Show percentages in language selector

[lukevella]

It would be useful to see what percentage of the project is translated in a language in the language selector. This might encourage users to contribute translations.

Example from Open Collective:

[kleenkanteen]

Can't every languge be translated from english in bulk using the google translate API? Or even a simple script? Any language errors can be fixed as users see them.

[fraserws]

Interested in having a look at this one, is this possible through the crowdin api? (https://developer.crowdin.com/api/v2/#operation/api.projects.branches.languages.progress.getMany)