In the Translations settings, the Preferred Languages list has a tool tip that reads in part: "If no other translations are available then US English will be used." The name of the language, however, is "American English" in that window (and in the files all_languages and kf5_all_languages). "US English" is a name for a locale. The language list should use consistent terminology to avoid confusion. To be clear: I'm not partial to either American English or US English and using both in different contexts is OK. Reproducible: Always Steps to Reproduce: Hold mouse pointer over the Preferred Languages list without clicking.
This is fixed in the rewritten Language KCM in 5.13.