Fix i18n issues

[vkbo]

Summary:

This PR solves two issues:

• All strings defined in the build settings core class were sent to the GUI untranslated even when translations exist.
• When no GUI language is set, novelWriter defaults to the system locale. However, if that is not available in novelWriter, the Preferences dialog would just leave the dropbox on the first option, which is currently "Deutch". The config class now only defaults to system locale if there is a translation file for it available. Otherwise it defaults to "British English". The Preferences will also default to "British English" if the language setting in config is invalid or has become unavailable.

Related Issue(s):

Closes #1563
Closes #1564

Reviewer's Checklist:

• The header of all files contain a reference to the repository license
• The overall test coverage is increased or remains the same as before
• All tests are passing
• All flake8 checks are passing and the style guide is followed
• Documentation (as docstrings) is complete and understandable
• Only files that have been actively changed are committed