[IP-531] Return to the application current language

[miquel-cabanas]

Pull Request Checklist

• My code follows the code formatting guidelines.
• I have an issue ID for this pull request.
• I selected the corresponding branch.
• I have rebased my changes on top of the corresponding branch.

Issue Type (Please check one or more)

• Bugfix

• Improvement of an existing Feature

• New Feature

  New Feature

When translating a string not available in the current application language, (1) save the current language setting before switching to the default system-language to translate the string, and (2) once the string has been translated, restore the application to its original language setting.

I have tested the fix without errors in a multilingual setup, with 3 different non-English languages for the system, user and client:

• all strings have been properly translated in the first language which included them according to the hierarchy: user language, system language and English; and
• quote and invoice pdf files have been created in the client-language.