Second attempt at fixing check-man

I was wrong, actually LANG-vs-LC_ALL only fixed one case, by mistake. To
get proper UTF-8 encoding, we need to enforce any UTF-8 locale. We
choose the 'default' of en_US.UTF-8.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>