Add email translations

Convert emails from Pug template to Handlebars because i18next doesn't
support Pug

server/core/controllers/api/client-config.ts

@@ -0,0 +1,36 @@
+import { is18nLocale } from '@peertube/peertube-core-utils'
+import { HttpStatusCode } from '@peertube/peertube-models'
+import { setClientLanguageCookie } from '@server/helpers/i18n.js'
+import express from 'express'
+import { apiRateLimiter } from '../../middlewares/index.js'
+
+const clientConfigRouter = express.Router()
+
+clientConfigRouter.use(apiRateLimiter)
+
+clientConfigRouter.post(
+  '/update-interface-language',
+  updateLanguage
+)
+
+// ---------------------------------------------------------------------------
+
+export {
+  clientConfigRouter
+}
+
+// ---------------------------------------------------------------------------
+
+function updateLanguage (req: express.Request, res: express.Response) {
+  const language = req.body.language
+
+  if (language !== null && !is18nLocale(language)) {
+    return res.fail({
+      message: req.t('{language} is not a valid language', { language })
+    })
+  }
+
+  setClientLanguageCookie(res, language)
+
+  return res.sendStatus(HttpStatusCode.NO_CONTENT_204)
+}