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.
I encourage you to read The Absolute Minimum Every Software Developer Absolutely, Positively Must Know About Unicode and Character Sets (No Excuses!)
é
isn't showing as a box or causing the system to crash or anything.[a-z 0-9 + # - .]
.