Adjust tox.ini checklang target

Test "all" languages so that we only need to adjust test-languages.sh if
new languages get added. For now there's only Japanese but let's do
it the same as other documentation repositories.

Change-Id: I3d95a8ab64cd45fe6a6dce7ad8885b67772c8385
Merge "Adjust tox.ini checklang target"