i18n: Fixed i18n_test by setting the LANGUAGE env variable

When I ran i18n_test, I had failures on 'ja_JP' and 'de_DE' tests. After
a little search, it turns out that gettext was unable to find the proper
mo files. cola's approach to setting up its translation is to set
environement variables and then call `translation()` without a
`languages` argument.

Before this patch, the environment variables we set were "LANG" and
"LC_MESSAGES". However, this is what Python doc for `find()` says:

"""
If languages is not given, then the following environment variables are
searched: LANGUAGE, LC_ALL, LC_MESSAGES, and LANG. The first one
returning a non-empty value is used for the languages variable.
"""

Therefore, non-empty "LANGUAGE" or "LC_ALL" env variables would
mess up our translation.

I've added a line setting "LANGUAGE" (in addition to the two others
already there) and it fixed the tests for me.

Signed-off-by: Virgil Dupras <hsoft@hardcoded.net>