Thank you for your response! Unfortunately this didn't make the trick, but helped me gather some information. When running: $ strace -o file.log flatpak run <
app.id>, I'm able to see some output referring to the locale I'm currently using:
openat(AT_FDCWD, "/usr/share/locale/en_GB.UTF-8/LC_MESSAGES/libc.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/usr/share/locale/en_GB.utf8/LC_MESSAGES/libc.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/usr/share/locale/en_GB/LC_MESSAGES/libc.mo", O_RDONLY) = 3
openat(AT_FDCWD, "/usr/share/locale/en_GB.UTF-8/LC_MESSAGES/flatpak.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/usr/share/locale/en_GB.utf8/LC_MESSAGES/flatpak.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/usr/share/locale/en_GB/LC_MESSAGES/flatpak.mo", O_RDONLY) = 4
openat(AT_FDCWD, "/usr/share/locale/en_GB.UTF-8/LC_MESSAGES/glib20.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/usr/share/locale/en_GB.utf8/LC_MESSAGES/glib20.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/usr/share/locale/en_GB/LC_MESSAGES/glib20.mo", O_RDONLY) = 4
While using a locale I didn't translate, the output is similar:
openat(AT_FDCWD, "/usr/share/locale/ja_JP.UTF-8/LC_MESSAGES/libc.mo", O_RDONLY) = -1 ENOENT (そのようなファイルやディレクトリはありません)
openat(AT_FDCWD, "/usr/share/locale/ja_JP.utf8/LC_MESSAGES/libc.mo", O_RDONLY) = -1 ENOENT (そのようなファイルやディレクトリはありません)
openat(AT_FDCWD, "/usr/share/locale/ja_JP/LC_MESSAGES/libc.mo", O_RDONLY) = -1 ENOENT (そのようなファイルやディレクトリはありません)
openat(AT_FDCWD, "/usr/share/locale/ja_JP.UTF-8/LC_MESSAGES/flatpak.mo", O_RDONLY) = -1 ENOENT (そのようなファイルやディレクトリはありません)
openat(AT_FDCWD, "/usr/share/locale/ja_JP.utf8/LC_MESSAGES/flatpak.mo", O_RDONLY) = -1 ENOENT (そのようなファイルやディレクトリはありません)
openat(AT_FDCWD, "/usr/share/locale/ja_JP/LC_MESSAGES/flatpak.mo", O_RDONLY) = -1 ENOENT (そのようなファイルやディレクトリはありません)
Unfortunately, I don't know how much this helps and the fact that it says "(No such file or directory)", doesn't help much... Do you know someone/somewhere else I can ask?