“Inter Milan”
The full name is Football Club Internazionale Milano. But the common name is Internazionale, or Inter for short. If you want an Inter fan to think you’re a moron, go ahead and call the team “Inter Milan”. It’s a little more accurate than the Sporting Lisbon mistake (because at least “Milano” is in the full team name) but they’ll still look at you with pity. Save yourself two syllables and stick with just “Inter”.

“AC Milan”
This is a little more borderline. It’s technically correct, as the full team name is Associazione Calcio Milan, or A.C. Milan for short. But think of it this way… do you always say the full Manchester United F.C.? Do you call Alex Ferguson’s team “FC” for short. Of course you don’t. So better to call this team Milan.