Leftover work discussed on T232410: Newcomer tasks: add language question to welcome survey

In T232410#6377157 @MMiller_WMF wrote:
[...] ...we should generally exclude already-selected languages from the list for subsequent selections, both on desktop and mobile.

Note: work in progress patch is at https://gerrit.wikimedia.org/r/c/mediawiki/extensions/MobileFrontend/+/620363/