I installed SMPlayer 20.4.2 (revision 9306) Qt 5.15.0 and SMTube 18.11.0 on Haiku.
English is used even though the system is using danish.
I can change to danish in preferences in SMPlayer but not in SMTube.
Seems if i set the language to danish and but to system language that it correctly uses danish.
But the first time after installation it does not seem to follow the system language.
Well the website content of SMTube is in danish but not the GUI.