As a follow-up to T105192 and to our latest weekly team meeting:
There's no convenient way on Special:CXStats to notice in which languages the trend went down, unless you are looking on the wiki in that language, and this doesn't scale to our number of languages.
The first simple thing that @Pginer-WMF suggested is to show a plus or minus next to each language bar on CXStats, but this could be developed further.