Localization refactoring:

* the front-end now use global constants, based on the translation key
* build-client.js use the ESBuild "define" directive to replace these
  globals at compile time, by the english value
* build:client must now be called after build:languages
* moving the loadLoc and loc backend functions in a separate lib

server/lib/loc.ts

@@ -0,0 +1,43 @@
+import { resolve } from 'path'
+import { existsSync, promises as fsPromises } from 'fs'
+
+const locContent: Map<string, string> = new Map<string, string>()
+
+/**
+ * Currently, the Peertube plugin system assumes that settings strings
+ * are localized in english, and will be translated on front-end.
+ * This system make it hard to have complex strings (with html, newlines, ...).
+ * See https://github.com/Chocobozzz/PeerTube/issues/4523
+ *
+ * Waiting for a better solution, we implemented a custom solution:
+ * - We are using keys to identify strings
+ * - the `loc` function gets the english segment for the key
+ * - the build-languages.js script builds all needed files.
+ * @param key The key to translate
+ */
+function loc (key: string): string {
+  return locContent.get(key) ?? key
+}
+
+async function loadLoc (): Promise<void> {
+  const filePath = resolve(__dirname, '..', '..', 'languages', 'en.reference.json')
+  if (!existsSync(filePath)) {
+    throw new Error(`File ${filePath} missing, can't load plugin loc strings`)
+  }
+  const content = await fsPromises.readFile(filePath, 'utf8')
+  const json = JSON.parse(content ?? '{}')
+  if (typeof json !== 'object') {
+    throw new Error(`File ${filePath} invalid, can't load plugin loc strings`)
+  }
+  for (const k in json) {
+    const v = json[k]
+    if (typeof v === 'string') {
+      locContent.set(k, v)
+    }
+  }
+}
+
+export {
+  loc,
+  loadLoc
+}