Closes #587. The issue points to the relevant sphinx source,
where it can be seen sphinx "marks" translated strings with <translated> at the
end of the source_path argument, which is then stored as the current_source attribute of
the docutils.nodes.document class, which is the second argument of Parser.parse.

This adds an extra condition to fall back to myst-parser which allows using myst-nb with translated
content.

It would be nice to add some test for this but I am not familiar enough with the test suite yet,
I'll try to add one in the coming days. It can probably reuse one of the existing notebooks,
but it will be necessary to add a locale folder with the translations, will it be ok to add
it directly in the tests folder?

So far I have only checked it works locally on a couple repos I had
(both executing and not executing notebooks).