Comment 1 for bug 1825644

Looks like QT parses the language name from the file name. No matter the language node in the *.ts file or the content of the *.qrc file.
https://github.com/mixxxdj/mixxx/pull/1937

Wonder if the mixed up source languages are another possible cause:
The .tx [1] file sets ``en_us`` as source language, while in fact the source language for the Mixxx application on transifex.com is ``en``. You can not change the source language for a project on Transifex [2]. The *.ts files source language is set to ``en`` too [3]

[1] https://github.com/mixxxdj/mixxx/blob/c9614d22e4d0b288821b908ec1944379683d7e5d/.tx/config#L8
[2] https://docs.transifex.com/faq/all#is-it-possible-to-change-my-projects-source-language
[3] https://github.com/mixxxdj/mixxx/blob/c9614d22e4d0b288821b908ec1944379683d7e5d/res/translations/mixxx.ts#L3