./run_tests.sh --makemessages update English PO files only

Closes-Bug: #1224388

We don't need to update PO files of all languages.
PO files of languages other than English should be update by
importing translations from Transifex.

This commit changes "run_tests.sh --makemessages" only updates
English PO files. This makes a patch updating PO files shorter
and helps reviewers to check it.

Change-Id: I5410a06b0bcb0f42c5a0853f95a3fe2087519d51
Merge "./run_tests.sh --makemessages update English PO files only"