Translate strings which we're sending to the frontend

[Cogitri]

It'd be nice if we were to translate strings such as error messages, otherwise frontends have to do that.

From what I can see there are two ways to go about this:

• Gettext, which would mean that we'd have to depend on a C lib. It's pretty advanced though and has very good support by translation tools, such as Weblate (which I use for Tau).
• Fluent.rs, which is being used by Firefox already (IIRC?) and is pure Rust. It's in its early days though and isn't well supported in translation tools yet.

We could also just ignore this and leave the translating to the frontends, e.g. in alert frontends can match the error code and send the localized message then.

[cmyr]

I would put this in the "one day" category, for now, but yes if core includes any human readable text it would be nice if core also provided localization.

[Cogitri]

Well, if we go the gettext route it'd be as easy as adding gettext(<string>) everywhere we have a string we want to translate and initializing gettext when starting xi-core