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
évolution into the tag list when creating a question, evolution will still be suggested. Likewise searching
evolution will show posts with either version, and searching
évolution will shows posts with either version. In sum, the accenting has no effect on the system at all.
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?