Default language is Japanese

I've tried to change the GUI language in many ways, but it only shows in Japanese.

LC_ALL=C flatpak run xyz.rust4diva.Rust4Diva 
Starting Rust4Diva Slint Edition
Detected locale "C" with character encoding "ANSI_X3.4-1968", which is not UTF-8.
Qt depends on a UTF-8 locale, and has switched to "C.UTF-8" instead.
If this causes problems, reconfigure your locale. See the locale(1) manual
for more information.
Parsing: English
Parsing: 1337
Parsing: 日本語
Parsing: español
Parsing: 简体中文
Attempting to find the Steam folder
Starting dmm url listener
Server running at rust4diva.sock
Loading mods from /home/sabri/mods
appling priority incase of new mods
Unable to get diva directory
Does the app run?
Mod pack successfully applied
QThreadStorage: entry 1 destroyed before end of thread 0x55a603cffce0
QThreadStorage: entry 0 destroyed before end of thread 0x55a603cffce0

Start by reproducing the issue with LC_ALL=C flatpak run xyz.rust4diva.Rust4Diva and inspect how the GUI selects its language. The report identifies the locale parsing and Flatpak launch as relevant entry points, but names no source files or tests. Done means the GUI no longer defaults to Japanese and the language can be changed as expected.