auto-detect composer language from keyboard

adds a new expo-keyboard-language native module that reads the current
keyboard language on iOS (via first responder's textInputMode) and
Android (via InputMethodManager). iOS also gets real-time updates when
the keyboard language changes.

users opt in via Settings > Languages > "Automatically detect post
language from keyboard" (native only, marked experimental). when
enabled, the composer language tracks the keyboard language. users can
override per-session by selecting a language manually, and re-enable
auto mode via the "Automatic" item in the language menu.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

modules/expo-keyboard-language/android/build.gradle

@@ -0,0 +1,39 @@
+apply plugin: 'com.android.library'
+
+group = 'expo.modules.keyboardlanguage'
+version = '1.0.0'
+
+def expoModulesCorePlugin = new File(project(":expo-modules-core").projectDir.absolutePath, "ExpoModulesCorePlugin.gradle")
+apply from: expoModulesCorePlugin
+applyKotlinExpoModulesCorePlugin()
+useCoreDependencies()
+useExpoPublishing()
+
+def useManagedAndroidSdkVersions = false
+if (useManagedAndroidSdkVersions) {
+  useDefaultAndroidSdkVersions()
+} else {
+  buildscript {
+    ext.safeExtGet = { prop, fallback ->
+      rootProject.ext.has(prop) ? rootProject.ext.get(prop) : fallback
+    }
+  }
+  project.android {
+    compileSdkVersion safeExtGet("compileSdkVersion", 34)
+    defaultConfig {
+      minSdkVersion safeExtGet("minSdkVersion", 21)
+      targetSdkVersion safeExtGet("targetSdkVersion", 34)
+    }
+  }
+}
+
+android {
+  namespace "expo.modules.keyboardlanguage"
+  defaultConfig {
+    versionCode 1
+    versionName "1.0.0"
+  }
+  lintOptions {
+    abortOnError false
+  }
+}

modules/expo-keyboard-language/android/src/main/java/expo/modules/keyboardlanguage/ExpoKeyboardLanguageModule.kt

@@ -0,0 +1,31 @@
+package expo.modules.keyboardlanguage
+
+import android.os.Build
+import android.view.inputmethod.InputMethodManager
+import androidx.core.content.getSystemService
+import expo.modules.kotlin.modules.Module
+import expo.modules.kotlin.modules.ModuleDefinition
+
+class ExpoKeyboardLanguageModule : Module() {
+  override fun definition() =
+    ModuleDefinition {
+      Name("ExpoKeyboardLanguage")
+
+      Function("getCurrentKeyboardLanguage") {
+        val context = appContext.reactContext ?: return@Function null
+        val imm = context.getSystemService<InputMethodManager>() ?: return@Function null
+        val subtype = imm.currentInputMethodSubtype ?: return@Function null
+
+        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
+          val tag = subtype.languageTag
+          if (tag.isNotEmpty()) return@Function tag
+        }
+
+        @Suppress("DEPRECATION")
+        val locale = subtype.locale
+        if (locale.isNotEmpty()) return@Function locale
+
+        return@Function null
+      }
+    }
+}

modules/expo-keyboard-language/expo-module.config.json

@@ -0,0 +1,9 @@
+{
+  "platforms": ["ios", "android"],
+  "ios": {
+    "modules": ["ExpoKeyboardLanguageModule"]
+  },
+  "android": {
+    "modules": ["expo.modules.keyboardlanguage.ExpoKeyboardLanguageModule"]
+  }
+}

modules/expo-keyboard-language/index.ts

@@ -0,0 +1,4 @@
+export {
+  addKeyboardLanguageListener,
+  getCurrentKeyboardLanguage,
+} from './src/index'

modules/expo-keyboard-language/ios/ExpoKeyboardLanguage.podspec

@@ -0,0 +1,20 @@
+Pod::Spec.new do |s|
+  s.name           = 'ExpoKeyboardLanguage'
+  s.version        = '1.0.0'
+  s.summary        = 'Expo module to detect the current keyboard language'
+  s.description    = 'Expo module to detect the current keyboard language'
+  s.author         = ''
+  s.homepage       = 'https://github.com/bluesky-social/social-app'
+  s.platforms      = { :ios => '13.4' }
+  s.source         = { git: '' }
+  s.static_framework = true
+
+  s.dependency 'ExpoModulesCore'
+
+  s.pod_target_xcconfig = {
+    'DEFINES_MODULE' => 'YES',
+    'SWIFT_COMPILATION_MODE' => 'wholemodule'
+  }
+
+  s.source_files = "**/*.{h,m,mm,swift,hpp,cpp}"
+end

modules/expo-keyboard-language/ios/ExpoKeyboardLanguageModule.swift

@@ -0,0 +1,75 @@
+import ExpoModulesCore
+
+public class ExpoKeyboardLanguageModule: Module {
+  public func definition() -> ModuleDefinition {
+    Name("ExpoKeyboardLanguage")
+
+    Events("onKeyboardLanguageChange")
+
+    Function("getCurrentKeyboardLanguage") {
+      return self.currentKeyboardLanguage()
+    }
+
+    OnStartObserving {
+      NotificationCenter.default.addObserver(
+        self,
+        selector: #selector(self.onInputModeOrResponderChange),
+        name: UITextInputMode.currentInputModeDidChangeNotification,
+        object: nil
+      )
+      NotificationCenter.default.addObserver(
+        self,
+        selector: #selector(self.onInputModeOrResponderChange),
+        name: UIResponder.keyboardDidShowNotification,
+        object: nil
+      )
+    }
+
+    OnStopObserving {
+      NotificationCenter.default.removeObserver(
+        self,
+        name: UITextInputMode.currentInputModeDidChangeNotification,
+        object: nil
+      )
+      NotificationCenter.default.removeObserver(
+        self,
+        name: UIResponder.keyboardDidShowNotification,
+        object: nil
+      )
+    }
+  }
+
+  private var lastLanguage: String?
+
+  @objc
+  private func onInputModeOrResponderChange() {
+    let language = currentKeyboardLanguage()
+    if language != lastLanguage {
+      lastLanguage = language
+      sendEvent("onKeyboardLanguageChange", [
+        "language": language as Any
+      ])
+    }
+  }
+
+  private func currentKeyboardLanguage() -> String? {
+    let keyWindow = UIApplication.shared.connectedScenes
+      .compactMap { $0 as? UIWindowScene }
+      .flatMap { $0.windows }
+      .first { $0.isKeyWindow }
+
+    return keyWindow?.findFirstResponder()?.textInputMode?.primaryLanguage
+  }
+}
+
+private extension UIView {
+  func findFirstResponder() -> UIView? {
+    if isFirstResponder { return self }
+    for subview in subviews {
+      if let found = subview.findFirstResponder() {
+        return found
+      }
+    }
+    return nil
+  }
+}

modules/expo-keyboard-language/src/index.native.ts

@@ -0,0 +1,26 @@
+import {type EventSubscription} from 'expo-modules-core'
+import {requireNativeModule} from 'expo-modules-core'
+
+type ExpoKeyboardLanguageModule = {
+  getCurrentKeyboardLanguage(): string | null
+  addListener(
+    eventName: 'onKeyboardLanguageChange',
+    listener: (event: {language: string | null}) => void,
+  ): EventSubscription
+}
+
+const NativeModule = requireNativeModule<ExpoKeyboardLanguageModule>(
+  'ExpoKeyboardLanguage',
+)
+
+export function getCurrentKeyboardLanguage(): string | null {
+  return NativeModule.getCurrentKeyboardLanguage()
+}
+
+export function addKeyboardLanguageListener(
+  cb: (language: string | null) => void,
+): EventSubscription {
+  return NativeModule.addListener('onKeyboardLanguageChange', event => {
+    cb(event.language)
+  })
+}

modules/expo-keyboard-language/src/index.ts

@@ -0,0 +1,11 @@
+import {type EventSubscription} from 'expo-modules-core'
+
+export function getCurrentKeyboardLanguage(): string | null {
+  return null
+}
+
+export function addKeyboardLanguageListener(
+  _cb: (language: string | null) => void,
+): EventSubscription {
+  return {remove() {}}
+}

src/lib/hooks/useKeyboardLanguage.ts

@@ -0,0 +1,30 @@
+import {useEffect, useState} from 'react'
+
+import {
+  addKeyboardLanguageListener,
+  getCurrentKeyboardLanguage,
+} from '../../../modules/expo-keyboard-language'
+
+function toBcp2(tag: string | null): string | null {
+  if (!tag) return null
+  return tag.split('-')[0].toLowerCase()
+}
+
+/**
+ * Returns the current keyboard language as a 2-letter code (e.g. "en"),
+ * updating in real-time on iOS.
+ */
+export function useKeyboardLanguage(): string | null {
+  const [language, setLanguage] = useState(() =>
+    toBcp2(getCurrentKeyboardLanguage()),
+  )
+
+  useEffect(() => {
+    const sub = addKeyboardLanguageListener(lang => {
+      setLanguage(toBcp2(lang))
+    })
+    return () => sub.remove()
+  }, [])
+
+  return language
+}

src/lib/hooks/useKeyboardLanguage.web.ts

@@ -0,0 +1,6 @@
+/**
+ * Web stub — keyboard language detection is not available on web.
+ */
+export function useKeyboardLanguage(): null {
+  return null
+}

src/screens/Settings/LanguageSettings.tsx

@@ -11,7 +11,7 @@ import {
 import {sanitizeAppLanguageSetting} from '#/locale/helpers'
 import {APP_LANGUAGES, LANGUAGES} from '#/locale/languages'
 import {useLanguagePrefs, useLanguagePrefsApi} from '#/state/preferences'
-import {atoms as a, web} from '#/alf'
+import {atoms as a, useTheme, web} from '#/alf'
 import {Admonition} from '#/components/Admonition'
 import {Button} from '#/components/Button'
 import {useDialogControl} from '#/components/Dialog'
@@ -21,6 +21,8 @@ import {PlusLarge_Stroke2_Corner0_Rounded as PlusIcon} from '#/components/icons/
 import * as Layout from '#/components/Layout'
 import * as Select from '#/components/Select'
 import {Text} from '#/components/Typography'
+import {IS_NATIVE} from '#/env'
+import {device, useStorage} from '#/storage'
 import * as SettingsList from './components/SettingsList'
 
 const DEDUPED_LANGUAGES = LANGUAGES.filter(
@@ -31,8 +33,12 @@ const DEDUPED_LANGUAGES = LANGUAGES.filter(
 type Props = NativeStackScreenProps<CommonNavigatorParams, 'LanguageSettings'>
 export function LanguageSettingsScreen({}: Props) {
   const {_} = useLingui()
+  const t = useTheme()
   const langPrefs = useLanguagePrefs()
   const setLangPrefs = useLanguagePrefsApi()
+  const [useKbLang = false, setUseKbLang] = useStorage(device, [
+    'useKeyboardLanguage',
+  ])
 
   const contentLanguagePrefsControl = useDialogControl()
 
@@ -234,6 +240,43 @@ export function LanguageSettingsScreen({}: Props) {
               />
             </View>
           </SettingsList.Group>
+          {IS_NATIVE && (
+            <>
+              <SettingsList.Divider />
+              <SettingsList.Group iconInset={false}>
+                <SettingsList.ItemText>
+                  <Trans>Post language</Trans>
+                </SettingsList.ItemText>
+                <Toggle.Item
+                  name="use_keyboard_language"
+                  label={_(
+                    msg`Automatically detect post language from keyboard`,
+                  )}
+                  value={useKbLang}
+                  onChange={value => setUseKbLang(value)}
+                  style={[a.w_full, a.gap_md]}>
+                  <Text style={[a.flex_1]}>
+                    <Trans>
+                      Automatically detect post language from keyboard
+                    </Trans>
+                  </Text>
+                  <Toggle.Platform />
+                </Toggle.Item>
+                <Text
+                  style={[
+                    a.text_xs,
+                    a.leading_snug,
+                    t.atoms.text_contrast_medium,
+                    a.mt_sm,
+                  ]}>
+                  <Trans>
+                    Experimental. When enabled, the composer will default to
+                    your keyboard language.
+                  </Trans>
+                </Text>
+              </SettingsList.Group>
+            </>
+          )}
         </SettingsList.Container>
       </Layout.Content>
     </Layout.Screen>

src/state/preferences/languages.tsx

@@ -12,7 +12,7 @@ type ApiContext = {
   setPrimaryLanguage: (code2: string) => void
   setPostLanguage: (commaSeparatedLangCodes: string) => void
   setContentLanguages: (code2s: string[]) => void
-  savePostLanguageToHistory: () => void
+  savePostLanguageToHistory: (resolvedLanguage?: string) => void
   setAppLanguage: (code2: AppLanguage) => void
 }
 
@@ -61,21 +61,28 @@ export function Provider({children}: React.PropsWithChildren<{}>) {
       /**
        * Saves whatever language codes are currently selected into a history array,
        * which is then used to populate the language selector menu.
+       *
+       * If `resolvedLanguage` is provided (e.g. from auto-detected keyboard
+       * language), it will be saved to history instead of the current
+       * `postLanguage` preference.
        */
-      savePostLanguageToHistory() {
+      savePostLanguageToHistory(resolvedLanguage?: string) {
         // filter out duplicate `this.postLanguage` if exists, and prepend
         // value to start of array
-        setStateWrapped(s => ({
-          ...s,
-          postLanguageHistory: [s.postLanguage]
-            .concat(
-              s.postLanguageHistory.filter(
-                commaSeparatedLangCodes =>
-                  commaSeparatedLangCodes !== s.postLanguage,
-              ),
-            )
-            .slice(0, 6),
-        }))
+        setStateWrapped(s => {
+          const langToSave = resolvedLanguage ?? s.postLanguage
+          return {
+            ...s,
+            postLanguageHistory: [langToSave]
+              .concat(
+                s.postLanguageHistory.filter(
+                  commaSeparatedLangCodes =>
+                    commaSeparatedLangCodes !== langToSave,
+                ),
+              )
+              .slice(0, 6),
+          }
+        })
       },
       setAppLanguage(code2: AppLanguage) {
         setStateWrapped(s => ({...s, appLanguage: code2}))

src/storage/schema.ts

@@ -61,6 +61,12 @@ export type Device = {
   activitySubscriptionsNudged?: boolean
   threadgateNudged?: boolean
 
+  /**
+   * Whether to automatically detect post language from the keyboard language.
+   * Experimental feature, opt-in.
+   */
+  useKeyboardLanguage?: boolean
+
   /**
    * Policy update overlays. New IDs are required for each new announcement.
    */

src/view/com/composer/Composer.tsx

@@ -70,6 +70,7 @@ import {
   type SupportedMimeTypes,
 } from '#/lib/constants'
 import {useIsKeyboardVisible} from '#/lib/hooks/useIsKeyboardVisible'
+import {useKeyboardLanguage} from '#/lib/hooks/useKeyboardLanguage'
 import {useNonReactiveCallback} from '#/lib/hooks/useNonReactiveCallback'
 import {useWebMediaQueries} from '#/lib/hooks/useWebMediaQueries'
 import {mimeToExt} from '#/lib/media/video/util'
@@ -134,6 +135,7 @@ import * as Toast from '#/components/Toast'
 import {Text} from '#/components/Typography'
 import {useAnalytics} from '#/analytics'
 import {IS_ANDROID, IS_IOS, IS_LIQUID_GLASS, IS_NATIVE, IS_WEB} from '#/env'
+import {device, useStorage} from '#/storage'
 import {BottomSheetPortalProvider} from '../../../../modules/bottom-sheet'
 import {
   draftToComposerPosts,
@@ -243,26 +245,52 @@ export const ComposePost = ({
     replyTo?.langs || [],
   )
 
+  /**
+   * Automatic keyboard language detection.
+   */
+  const keyboardLanguage = useKeyboardLanguage()
+  const [autoLangEnabled = false] = useStorage(device, ['useKeyboardLanguage'])
+  const [manualOverride, setManualOverride] = useState(false)
+  const isAutoMode = !!autoLangEnabled && !manualOverride
+
   /**
    * The currently selected languages of the post. Prefer local temporary
-   * language suggestion over global lang prefs, if available.
+   * language suggestion, then auto-detected keyboard language, then global
+   * lang prefs.
    */
   const currentLanguages = useMemo(
     () =>
       acceptedLanguageSuggestion
         ? [acceptedLanguageSuggestion]
-        : toPostLanguages(langPrefs.postLanguage),
-    [acceptedLanguageSuggestion, langPrefs.postLanguage],
+        : isAutoMode && keyboardLanguage
+          ? [keyboardLanguage]
+          : toPostLanguages(langPrefs.postLanguage),
+    [
+      acceptedLanguageSuggestion,
+      isAutoMode,
+      keyboardLanguage,
+      langPrefs.postLanguage,
+    ],
   )
 
   /**
    * When the user selects a language from the composer language selector,
    * clear any temporary language suggestions they may have selected
-   * previously, and any we might try to suggest to them.
+   * previously, and any we might try to suggest to them. Also locks out
+   * auto mode for this session.
    */
   const onSelectLanguage = () => {
     setAcceptedLanguageSuggestion(null)
     setReplyToLanguages([])
+    setManualOverride(true)
+  }
+
+  /**
+   * Re-enable auto mode when the user taps "Automatic" in the language menu.
+   */
+  const onSelectAutomatic = () => {
+    setAcceptedLanguageSuggestion(null)
+    setManualOverride(false)
   }
 
   const [composerState, composerDispatch] = useReducer(
@@ -939,7 +967,7 @@ export const ComposePost = ({
         originalLocalRefs: composerState.originalLocalRefs,
       })
     }
-    setLangPrefs.savePostLanguageToHistory()
+    setLangPrefs.savePostLanguageToHistory(fromPostLanguages(currentLanguages))
     if (initQuote) {
       // We want to wait for the quote count to update before we call `onPost`, which will refetch data
       whenAppViewReady(agent, initQuote.uri, res => {
@@ -1122,6 +1150,10 @@ export const ComposePost = ({
         }}
         currentLanguages={currentLanguages}
         onSelectLanguage={onSelectLanguage}
+        onSelectAutomatic={onSelectAutomatic}
+        keyboardLanguage={keyboardLanguage}
+        isAutoMode={isAutoMode}
+        autoLangEnabled={autoLangEnabled}
         openGallery={openGallery}
       />
     </>
@@ -1823,6 +1855,10 @@ function ComposerFooter({
   onAddPost,
   currentLanguages,
   onSelectLanguage,
+  onSelectAutomatic,
+  keyboardLanguage,
+  isAutoMode,
+  autoLangEnabled,
   openGallery,
 }: {
   post: PostDraft
@@ -1834,6 +1870,10 @@ function ComposerFooter({
   onAddPost: () => void
   currentLanguages: string[]
   onSelectLanguage?: (language: string) => void
+  onSelectAutomatic?: () => void
+  keyboardLanguage: string | null
+  isAutoMode: boolean
+  autoLangEnabled: boolean
   openGallery?: boolean
 }) {
   const t = useTheme()
@@ -1992,6 +2032,10 @@ function ComposerFooter({
         <PostLanguageSelect
           currentLanguages={currentLanguages}
           onSelectLanguage={onSelectLanguage}
+          onSelectAutomatic={onSelectAutomatic}
+          keyboardLanguage={keyboardLanguage}
+          isAutoMode={isAutoMode}
+          autoLangEnabled={autoLangEnabled}
         />
         <CharProgress
           count={post.shortenedGraphemeLength}

src/view/com/composer/select-language/PostLanguageSelect.tsx

@@ -21,9 +21,17 @@ import {Text} from '#/components/Typography'
 export function PostLanguageSelect({
   currentLanguages: currentLanguagesProp,
   onSelectLanguage,
+  onSelectAutomatic,
+  keyboardLanguage,
+  isAutoMode,
+  autoLangEnabled,
 }: {
   currentLanguages?: string[]
   onSelectLanguage?: (language: string) => void
+  onSelectAutomatic?: () => void
+  keyboardLanguage?: string | null
+  isAutoMode?: boolean
+  autoLangEnabled?: boolean
 }) {
   const {_} = useLingui()
   const langPrefs = useLanguagePrefs()
@@ -46,26 +54,7 @@ export function PostLanguageSelect({
     onSelectLanguage?.(langsString)
   }
 
-  if (
-    dedupedHistory.length === 1 &&
-    dedupedHistory[0] === langPrefs.postLanguage
-  ) {
-    return (
-      <>
-        <LanguageBtn onPress={languageDialogControl.open} />
-        <LanguageSelectDialog
-          titleText={<Trans>Choose post languages</Trans>}
-          subtitleText={
-            <Trans>Select up to 3 languages used in this post</Trans>
-          }
-          control={languageDialogControl}
-          currentLanguages={currentLanguages}
-          onSelectLanguages={onSelectLanguages}
-          maxLanguages={3}
-        />
-      </>
-    )
-  }
+  const showAutoItem = !!autoLangEnabled && !!keyboardLanguage
 
   return (
     <>
@@ -77,6 +66,19 @@ export function PostLanguageSelect({
         </Menu.Trigger>
         <Menu.Outer>
           <Menu.Group>
+            {showAutoItem && (
+              <Menu.Item
+                label={_(msg`Automatic`)}
+                onPress={() => onSelectAutomatic?.()}>
+                <Menu.ItemText>
+                  <Trans>Automatic</Trans>
+                  {' ('}
+                  {codeToLanguageName(keyboardLanguage, langPrefs.appLanguage)}
+                  {')'}
+                </Menu.ItemText>
+                <Menu.ItemRadio selected={!!isAutoMode} />
+              </Menu.Item>
+            )}
             {dedupedHistory.map(historyItem => {
               const langCodes = historyItem.split(',')
               const langName = langCodes
@@ -92,7 +94,9 @@ export function PostLanguageSelect({
                   }}>
                   <Menu.ItemText>{langName}</Menu.ItemText>
                   <Menu.ItemRadio
-                    selected={currentLanguages.includes(historyItem)}
+                    selected={
+                      !isAutoMode && currentLanguages.includes(historyItem)
+                    }
                   />
                 </Menu.Item>
               )