grub.cfg takes it lang setting from the LANG variable even if an
overriding value has been set with LC_MESSAGES.
People like me that want's the system to talk english but handle
swedish dates etc. may use settings like this:
$ cat /etc/default/locale
LANG=sv_SE.UTF-8
LC_MESSAGES=en_US.UTF-8

Currently update-grub will set lang to 'sv' instead of 'en' in this
case.
In my opinion grub should use the same language as other commands in
the same installation.

I have attached a patch that I use to change the behavior.

/Andreas

Dear grub Maintainers,

four years after being reported (with a patch!), this bug is still open.
I would appreciate if you could kindly review the provided patch, which
looks like a simple enough fix to make grub follow common practice on
Linux wrt. the locale (in particular, LC_MESSAGES overriding LANG for,
well, messages).

Kind regards,
Ralf