The language selection box is a standard multi-select control, as implemented by your browser. On desktop that means it’s likely a flat list with multiple items you can select, on mobile you’ll probably get a popup with checkboxes. These controls used to be ubiquitous, but given the amount of questions they seem to prompt, I suppose they fell out of favor since mobile phones became more popular.

Assuming you’re on desktop, the list is controlled like file selection in Windows explorer (and probably others) is controlled. Clicking an item in the list selects just that item. Holding down the control key allows multiple items to be selected. Holding down shift allows selecting a range, starting from an already selected item.

See this article for some screenshots if this is still unclear. Multi selection will work in other programs as well!

Unlike Windows Explorer, ctrl+A doesn’t work. To select all, click the first item in the list, scroll all the way down, and shift+click the last item in the list.

When you’re selecting languages, try holding ctrl when clicking the ones you want. It should allow you to select more than one.