Booting with grub2 there is for few seconds an error message at the screen: Error: file '/boot/grub2/locale/en.mo.gz' not found. There is a lot of discussion in the web about this error message, but I could not found a real solution. The best I found was to set GRUB_TERMINAL_OUTPUT=console in /etc/default/grub. Thus the error message disappears, but you loose the graphical resolution at the grub menu too.
The message is harmless. If you really dislike it, set LANG=C when you run grub2-mkconfig, like so. LANG=C grub2-mkconfig -o /boot/grub2/grub.cfg *** This bug has been marked as a duplicate of bug 408599 ***