Clicking on languages in the list just selects them, with no “toggled on/off” indicator. The selection highlight is obviously not a “toggled” indicator, since you can only select one language at a time this way.
It’s a multi-selectable list but you need to Ctrl-click (on Windows; presumably Cmd-click on Mac?) to get a multiple selection. It then behaves as ‘expected’ (highlighted lines are selected), although this behaviour is not intuitive.
So you can nuke your multi-selection by misclicking?
Having just tried it - yes, a single-click on any option in the list sets the selection to just that one.
This is usual behaviour, in my experience - not something unique to Lemmy/Jerboa/etc.
They’re a relatively shitty UI design element, I think the general rule of thumb is to have a few items in the list, not ‘dozens’