[MM-60792] Add support for loading experimental (e.g., in progress) languages (#29009)

* Add support for loading experimental (e.g., in progress) languages

* Fix e2e config

* Improve language labels

* Improvements

webapp/scripts/gen_lang_imports.mjs

@@ -0,0 +1,62 @@
+#!/bin/node
+
+import * as fs from 'fs';
+import langmap from '../channels/src/i18n/langmap.js';
+
+let lines = '';
+const langIDs = [];
+const langFiles = {};
+const langLabels = {};
+
+fs.readdirSync('./channels/src/i18n').forEach(file => {
+  if (file.endsWith('.json')) {
+    const langID = file.substr(0, file.indexOf('.json'));
+
+    // Skip default language
+    if (langID === 'en') {
+      return
+    }
+    
+    langIDs.push(langID);
+    lines += `import ${langID.replace('-', '')} from './${file}';\n`;
+    langFiles[langID] = langID.replace('-', '');
+
+    let m = langmap[langID]
+    if (!m) {
+      // We fallback to the language code prefix if we can't find a map.
+      const id = langID.includes('-') ? langID.substr(0, langID.indexOf('-')) : langID;
+      for (const k of Object.keys(langmap)) {
+        if (k.startsWith(id)) {
+          m = langmap[k];
+          break;
+        }
+      }
+    }
+
+    langLabels[langID] = m ? m["nativeName"] : langID;
+  }
+});
+
+lines += `\nexport const langIDs = ${JSON.stringify(langIDs)};\n`
+lines += `\nexport const langLabels = ${JSON.stringify(langLabels)};\n`
+
+// To generate the file exports we need to do a bit more work to handle ids with dashes and also output a map of literals rather than strings.
+lines += '\nexport const langFiles = {' + Object.keys(langFiles).reduce((out, id, idx) => {
+  if (id.includes('-')) {
+    out += `'${id}':${langFiles[id]}`;
+  } else {
+    out += `${langFiles[id]}`;
+  }
+
+  if (idx !== (langIDs.length - 1)) {
+    out += ',';
+  }
+
+  return out;
+}, '') + '};';
+
+const header = `// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.\n\n// DO NOT EDIT - This file is generated by gen_lang_imports through the gen-lang-imports script\n\n/* eslint-disable */\n\n`;
+
+fs.writeFileSync('./channels/src/i18n/imports.ts', header+lines);
+