After reading a bit of comments, found the suggested solution to add lang=<country_code> as executable worked.
My solution has been to edit the global .desktop file (located at /usr/share/applications on Archlinux)
from:
Exec=/usr/bin/obsidian %U
to:
Exec=/bin/sh -c "/usr/bin/obsidian --lang=ja %U"
and also had to eliminate the one at $HOME/.local/share/applications as it was creating a duplicate under xfce4.
