See https://incubator.wikimedia.org/wiki/Special:RecentChanges (the "Filter translations" is T159795: Port Translate to new RC Filters system):

Incubator is a special wiki where wikis are prepared/tested to see if they make sense to spin into a real wiki.

For example, a new language Wikipedia is first created there before becoming a full Wikipedia. This means you need to be able to filter to see only changes from that soon-to-be-project. This is done with prefixes, e.g. https://incubator.wikimedia.org/wiki/Wp/atj/Kinocew (the part before the last slash is the prefix).

The UI shows this as a drop-down for family (Wikipedia, Wikibooks, Wiktionary, etc.), plus a free-form for language code. However, the free-form could be converted into a drop-down of languages, with configuration (they may not all be languages recognized by MW yet, need to check this)

This is a bit similar to the trailer parameter of CleanChanges (not-WMF deployed, though it briefly was, but see T56203: Core-ify the CleanChanges extension's intent (to filter and declutter RecentChanges)).