fix(l10n): normalize locale when fetching client.json

Fixes #1024.

To test this locally, you'll have to `rm -rf app/bower_components && bower install && grunt l10n-create-json` because the test depends on an updated en_US `.po` file.

/cc @shane-tomlinson @nchapman @pdehaan @jrgm 

Want to r? @pdehaan?