On the newly created French Language & Usage (private beta started today), I tried to have a tag named évolution, but the accent on the é is lost. The SE engine really should allow accents on tags.
If you type
I can see why you might not like it, but consider that tags already replace spaces with hypens and upper case letters with lower case letters. Tags aren't supposed to accurately represent proper names, accented letters, symbols, etc; they're supposed to be both useful and simple, and they are. Why complicate it?