I cannot choose another language, some weird css/html issue happens that hides all other fields. Looks like an overflow:hidden hides the possible selections.
It has to do (I think) with Dash's columnar layout which unfolds the menu over the next few columns at least in Chrome.
The quick workaround I found was to type out the language until it appears below and click on it or finish typing, then press enter. This should select it.
I'd love to hear from other Dash developers who've had, and solved this issue.