Changes:
* Use recommended build package for building as the use of setup.py as a
command line to tool is deprecated
* Install multiple packages with one command which theoretically saves
some time
* validate_po.py: Fix typo "varialbes"
* validate_po.py: Exit with code 1 on error
* validate_po.py: Process locales in alphabetical order
* validate_po.py: Report on progress
* Make GitHub Actions check translation files
Tero Paloheimo
Sebastian Pipping
Gobinath Loganathan