[Feature] Language dropdown

[TorstenDittmann]

For the cloud export right now we have an array with some languages to select from.

https://github.com/TorstenDittmann/OmniaWrite/blob/master/src/routes/Export/Cloud.svelte#L50-L55

A good addition would be implementing a library containing all the languages + 2-letter codes.

Sadly this is not supported by Firefox yet, so a library like globalize or i18n-iso-languages might be the best.

This is blocked by #89