localize invite-pages

[r10s]

currently, they're english only.

as important parts anyway depend on javascript, it'd be find if translations rely on javascript as well

[link2xt]

It uses https://www.i18next.com/
From https://www.i18next.com/overview/getting-started it seems that it should detect the language automatically.

Maybe removing

invite/index.html

Line 326 in e78fbc3

lng: (new URLSearchParams(window.location.search)).get('lang'),

is enough? I don't know how this is supposed to work, but for me (new URLSearchParams(window.location.search)).get('lang') typed into the console in Firefox returns null.

EDIT: actually URLSearchParams works like this: https://i.delta.chat/?lang=fr, https://i.delta.chat/?lang=ru
This code simply takes the language from the URL, but nobody passes it there.

EDIT: removing is not enough, but we can add language detector script.

[link2xt]

Made a PR to add language detector: #41

[link2xt]

Fixed with #41 and #42