Localized help urls:

Links to documentation are now using the front-end language to point to the translated documentation page
(except for some links generated from the backend, in the diagnostic tool for example).

Partial fix for #224.

client/utils/help.ts

@@ -0,0 +1,37 @@
+import type { RegisterClientOptions } from '@peertube/peertube-types/client'
+import { helpUrl } from 'shared/lib/help'
+
+/**
+ * Returns the current language code.
+ * This is the code that correspond to the path for localized documentation
+ * (in other words: 'fr', not 'fr-FR')
+ * @param clientOptions client options
+ * @returns language code
+ */
+async function getLangCode ({ peertubeHelpers }: RegisterClientOptions): Promise<string> {
+  // See the build-languages.js script for more information about this trick.
+  const s = await peertubeHelpers.translate('_language')
+  if (s === '_language') {
+    return 'en'
+  }
+  return s
+}
+
+/**
+ * Returns the url of the documentation, in the current user language.
+ * @param clientOptions client options
+ */
+async function localizedHelpUrl (
+  clientOptions: RegisterClientOptions,
+  helpUrlOptions: Parameters<typeof helpUrl>[0]
+): Promise<string> {
+  const lang = (await getLangCode(clientOptions)).toLowerCase() // lowercase is needed for zh-hans for example
+  if (lang === 'en') {
+    return helpUrl(helpUrlOptions)
+  }
+  return helpUrl(Object.assign({}, helpUrlOptions, { lang }))
+}
+
+export {
+  localizedHelpUrl
+}