Add internationalization framework and compile translation strings

[Jaifroid]

I began internatinalization of upstream (Kiwix JS) first because there is much less text overall in the upstream app, and also to mark the 10th anniversary. However, the PWA is increasingly used either as a quick "idiotproof" way of viewing a ZIM's content, or as a fallback for file picking where some mainstream apps have limitations. So it would make sense to internationalize it as well.

This will be a much slower task, because this app has many more functions and dialogue boxes associated with them. Upstream already has some 226 strings, and this app could easily have double. Just compiling the list of English-language strings will be a major job, let alone getting them translated... ☹️