Fix posting for special interface languages

Easy Japanses (ja_easy) and traditional Chinses (zh_Hant) use
(custom) non-ISO codes in the interface. Because MastoAPI only accepts
ISO 639 codes, the backend will return an error rendering users
unable to do anything unless the post’s language was explicitly set.

src/components/post_status_form/post_status_form.js

@@ -62,6 +62,13 @@ const deleteDraft = (draftKey) => {
 	localStorage.setItem('drafts', JSON.stringify(draftData));
 }
 
+const interfaceToISOLanguage = (ilang) => {
+    const sep = ilang.indexOf("_");
+    return sep < 0 ?
+        ilang :
+        ilang.substr(0, sep);
+}
+
 const PostStatusForm = {
   props: [
     'statusId',
@@ -140,6 +147,8 @@ const PostStatusForm = {
 
     const { postContentType: contentType, sensitiveByDefault, sensitiveIfSubject, interfaceLanguage, alwaysShowSubjectInput } = this.$store.getters.mergedConfig
 
+    const isoLanguage = interfaceToISOLanguage(interfaceLanguage)
+
     let statusParams = {
       spoilerText: this.subject || '',
       status: statusText,
@@ -149,7 +158,7 @@ const PostStatusForm = {
       poll: {},
       mediaDescriptions: {},
       visibility: this.suggestedVisibility(),
-      language: interfaceLanguage,
+      language: isoLanguage,
       contentType
     }
 
@@ -164,7 +173,7 @@ const PostStatusForm = {
         poll: this.statusPoll || {},
         mediaDescriptions: this.statusMediaDescriptions || {},
         visibility: this.statusScope || this.suggestedVisibility(),
-        language: this.statusLanguage || interfaceLanguage,
+        language: this.statusLanguage || isoLanguage,
         contentType: statusContentType
       }
     }