Decide if grades export should use localised decimal/thousands separators

While working on MDL-73824, it was detected that grades export always generate the files using English (dot) decimal separator.

This may sound reasonable, but I'm not 100% sure, so better report it as a new issue to be discussed / decided.

So I've skipped these 2 features from my tests in the linked issue:

• grade/export/txt/tests/behat/export.feature
• grade/export/xml/tests/behat/export.feature

And the point here is to:

1. Discuss and decide if grades exports (all formats) should observe localised number formatting.
2. Analyse the impact in different formats.
3. Consider if, maybe, that should be an export option.
4. Implement the decided approach
5. Cover it with tests, so different decimal/thousand separators are checked in the resulting files.

Ciao