Explicitely set LANGUAGE=C to avoid possible locale issues

In github issue #48 the following was reported when running with
option --debopt "--verbose --include=grub-pc,locales":

| *** update-locale: Error: invalid locale settings:  LANGUAGE=en_US:en LANG=en_US.UTF-8

While this shouldn't be a real problem but just a warning message
it still might be a good idea to explictely reset LANGUAGE=C.

See https://www.gnu.org/software/gettext/manual/html_node/Locale-Environment-Variables.html
for details regarding LANG/LC_ALL/LANGUAGE handling.

Fixes #48 @ github