Compare https://incubator.wikimedia.org/wiki/Special:Contributions/QuincyMcc?uselang=en and https://incubator.wikimedia.org/wiki/Special:Contributions/QuincyMcc?uselang=de
and notice the missing content of MediaWiki:Sp-contributions-footer.
On other wikis (meta, en) this problem does not seem to exist.
Copied from duplicate T148916: Language fallback does not work (anymore?) for AbuseFilter warnings.
On ptwiki, pt:Special:AbuseFilter/92 is supposed to warn¹ new users that attempt to upload files about the local policies. However, if the user set the language to pt-BR² in his preferences, the extension shows only a generic warning³ which do not explains anything.
I hope this is only a regression from some refactoring in the way fallback works, otherwise most of our users never got a decent warning message when they were hit by some edit filter...