kbuild: Unset language specific variables in setlocalversion script

This patch allows the use of setlocalversion script regardless of the language
parameters. Otherwise, the `svn info 2>/dev/null | grep '^Last Changed Rev'`
returns nothing because for instance, in French the text 'Last Changed Rev'
is replaced by 'Révision de la dernière modification'

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michal Marek <mmarek@suse.cz>