Is there enough of a fundamental difference between "do you want to sit" and "do you want to sit down" for it to be flagged?
Hmm, well in we do say "Eistedd i lawr" which is the accurate translation of "To sit down", so I imagine it's not accepted because the contributors didn't think about it or it's because there is a slight difference in the translation.
This is a zombie.
eistedd means 'sitting' or 'sitting down'. People do tend to add i lawr to emphasise 'down', but there is no strict need for that in either English or Welsh. Similary sefyll means both 'standing' and 'standing up'. It is down to personal usage and style, though.