Make membership page strings translatable

The membership page strings are not translatable. We should make them so, there may be some issues with the embedded html links and others, because it is a template.

See for example: https://gitlab.torproject.org/tpo/web/tpo/-/blob/master/templates/download.html

Also: the pictures are not appearing on the locales that are not English, see for example: https://www.torproject.org/es/about/membership/