[APP-718] Improvements and fixes to language handling (#931)

* Add locale helpers for narrowing languages

* Add a translate link to posts in a different language

* Update language filtering to use narrowing when multiple declared

* Fix a few more RTL layout cases

* Fix types

src/locale/helpers.ts

@@ -0,0 +1,81 @@
+import {AppBskyFeedDefs, AppBskyFeedPost} from '@atproto/api'
+import lande from 'lande'
+import {hasProp} from 'lib/type-guards'
+import * as bcp47Match from 'bcp-47-match'
+import {LANGUAGES_MAP_CODE2, LANGUAGES_MAP_CODE3} from './languages'
+
+export function code2ToCode3(lang: string): string {
+  if (lang.length === 2) {
+    return LANGUAGES_MAP_CODE2[lang]?.code3 || lang
+  }
+  return lang
+}
+
+export function code3ToCode2(lang: string): string {
+  if (lang.length === 3) {
+    return LANGUAGES_MAP_CODE3[lang]?.code2 || lang
+  }
+  return lang
+}
+
+export function getPostLanguage(
+  post: AppBskyFeedDefs.PostView,
+): string | undefined {
+  let candidates: string[] = []
+  let postText: string = ''
+  if (hasProp(post.record, 'text') && typeof post.record.text === 'string') {
+    postText = post.record.text
+  }
+
+  if (
+    AppBskyFeedPost.isRecord(post.record) &&
+    hasProp(post.record, 'langs') &&
+    Array.isArray(post.record.langs)
+  ) {
+    candidates = post.record.langs
+  }
+
+  // if there's only one declared language, use that
+  if (candidates?.length === 1) {
+    return candidates[0]
+  }
+
+  // no text? can't determine
+  if (postText.trim().length === 0) {
+    return undefined
+  }
+
+  // run the language model
+  let langsProbabilityMap = lande(postText)
+
+  // filter down using declared languages
+  if (candidates?.length) {
+    langsProbabilityMap = langsProbabilityMap.filter(
+      ([lang, _probability]: [string, number]) => {
+        return candidates.includes(code3ToCode2(lang))
+      },
+    )
+  }
+
+  if (langsProbabilityMap[0]) {
+    return code3ToCode2(langsProbabilityMap[0][0])
+  }
+}
+
+export function isPostInLanguage(
+  post: AppBskyFeedDefs.PostView,
+  targetLangs: string[],
+): boolean {
+  const lang = getPostLanguage(post)
+  if (!lang) {
+    // the post has no text, so we just say "yes" for now
+    return true
+  }
+  return bcp47Match.basicFilter(lang, targetLangs).length > 0
+}
+
+export function getTranslatorLink(lang: string, text: string): string {
+  return encodeURI(
+    `https://translate.google.com/?sl=auto&tl=${lang}&text=${text}`,
+  )
+}

src/locale/languages.ts

@@ -555,3 +555,7 @@ export const LANGUAGES_MAP_CODE2 = Object.fromEntries(
 export const LANGUAGES_MAP_CODE3 = Object.fromEntries(
   LANGUAGES.map(lang => [lang.code3, lang]),
 )
+// some additional manual mappings (not clear if these should be in the "official" mappings)
+if (LANGUAGES_MAP_CODE2.fa) {
+  LANGUAGES_MAP_CODE3.pes = LANGUAGES_MAP_CODE2.fa
+}