Add language setting to Project Settings

[vkbo]

Summary:

This PR moves the project language setting from the Manuscript Build tool to Project Settings where it belongs. This setting is now also used for the content in the Document Viewer.

Related Issue(s):

Closes #1588

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