Add lang in url

[pyphilia]

Add lang in url: This will be used to directly set the language in the member when he registers, and send a mail in a "correct" language.

Or lambda to detect from request.

[spaenleh]

Also displaying the interface in the correct language !

[pyphilia]

@spaenleh done?

[spaenleh]

What has been implemented in #321 is the ability to send the selected language to the backend.
We are still missing a way to:

1. boot the interface in the preferred language of the user (for example browsing the library in french, the signIn should link to auth in french)
2. Maybe infer the language based on browser if not supplied in the url

1 could be done easily with a query param that sets i18n
2 would need a bit more work

We also need to ensure that everything that links to auth sets the query param for the language in order to make the experience consistent.