Use Async callback for i18n

At the moment, all i18n is done via a sync callback. This can be resolved in two ways:

1. Request all i18n requests in one call, which will still be sync, but will be significantly faster
2. Use callbacks to update the text after it's displayed (or, before). This can use browser storage to speed up the display, as only the first one will be slow. Then you'd have a function that checks browser storage, and if it's there, returns that, and THEN does a request for gettext and updates browser storage (and the page) if the text has changed.