If you’re talking about the Lemmy Web UI and the list of languages that you want to see posts in, hold Control while clicking multiple items in the list. That’s a long-standing GUI convention for selecting multiple items in a list; it’ll work in other software too.

EDIT: That did make me curious how mobile browsers handle multiple select lists, since a soft keyboard won’t let you hold a key at the same time as you’re tapping in the webpage portion of the screen, and not all mobile users are going to have a physical keyboard attached. It looks like Firefox for Android just pops up a list with checkboxes as soon as you tap on a multiple select list:

https://www.w3schools.com/TAGS/tryit.asp?filename=tryhtml_select_multiple