collaborative branch to fix #775

Work to troubleshoot and fix #775. I vaguely remember we had to do some hacks to set up the site.active_lang variable, which is clearly related to the bug (see https://gitlab.com/fdroid/fdroid-website/-/blob/master/_includes/sidebar-search.html). So many those hacks are obsolete with newer versions of the polyglot plugin?

@linsui @seabass-labrax as discussed in today's meeting, this branch will be deployed to staging.f-droid.org until we fix #775. Feel free to force push to it, etc.