I have no strong opinion about doing this, but if it is done, I think defaulting to the last language the user used makes the most sense. If I have to fill it in every time, I probably simply won't recall/take the time most days, but if I fill it in once as English and it then remembers for me every time - then all of mine would end up tagged.

I'm not sure how big of an inconvenience that would be for people who post in multiple languages, or whether it might lead to mis-tagging, but for those of us posting in only one it would be a nice convenience. I'm not sure how to weigh those two needs against each other, though.