ENH: Improve logic searching translation files for custom app

[Thibault-Pelletier]

• Change Slicer_LANGUAGES from a GLOBAL property to a CACHE variable
  allowing language generation to be modified in the context of custom
  applications.

• Change behavior for missing translation files when disabling translation
  updates to allow custom applications to translate subparts of the
  application only.

• Fix qSlicerCoreApplication::loadTranslations to find generic forms of TS
  files when present.

[lassoan]

The content looks good to me. I have a few concerns about coding style - see inline comments.

[jcfr]

Thanks for the contribution 👍

Few nitpicks and this will be good for integration

If you some more bandwidth, another pull request that would need a little more work is this one: Slicer/SlicerGitSVNArchive#1162

[lassoan]

@Thibault-Pelletier Thanks again for your contribution. Would you like us to fix this up or you would be available to do it?

[Thibault-Pelletier]

Hi everyone,
Thank you very much for the feedbacks, I will integrate these changes and update my commit (hopefully by today or tomorrow at latest).

@jcfr the Slicer/SlicerGitSVNArchive#1162 PR seems interesting. We are currently tackling this type of functionalities and for now we are using PyQt5 lupdate executable to parse the python files for translation methods and the following decorator to make the widgets translatable while capturing the context :

def translatable(cls):
  """Decorator to add translation support to the decorated class.

  Decorator needs to be used for both Qt widgets and other widgets to allow translation to work properly
  """
  def tr(self, name):
    return qt.QCoreApplication.translate(self.__class__.__name__, name)

  setattr(cls, 'tr', tr)
  return cls

[jcfr]

I will further update the topic considering changes made in Slicer/SlicerGitSVNArchive#1162

[lassoan]

This is now merged in cd5cdbd.
We plan to implement lots of i18n improvements (for scripted modules, extensions, etc.) in the coming months.