Language selection fixes (#2917)

* Fix duplicated language entries from system and app language sets.
Closes #2900

* Prefer modern language codes.
Closes #2903

* Synchronize per-account default posting language with server.
Closes #2902

* Allow users to post in languages android doesn't know about yet (e.g. toki pona)

* Always put the preselected language at the top of the list

app/src/main/java/com/keylesspalace/tusky/db/AccountManager.kt

@@ -154,6 +154,7 @@ class AccountManager @Inject constructor(db: AppDatabase) {
             it.displayName = account.name
             it.profilePictureUrl = account.avatar
             it.defaultPostPrivacy = account.source?.privacy ?: Status.Visibility.PUBLIC
+            it.defaultPostLanguage = account.source?.language ?: ""
             it.defaultMediaSensitivity = account.source?.sensitive ?: false
             it.emojis = account.emojis ?: emptyList()