Why are all my games Japanese?

I'm a bit of an outsider when it comes to the Neo Geo scene, I bought my AES with 12 games about 6 months ago at a pawn shop(partly because I knew I could turn it out for a bigger profit than what I paid, that and King of the Monsters 2 was part of the deal, and I loved that game on my SNES).

Anyways, flash forward to the present and I'm preparing my games to sell on ebay. I have been using the pictures provided in the pricing guide to determine which game is the Japanese Version and which is the American version, but this is where things get confusing... everytime I boot up my AES it says "not meant for play outside of Japan etc etc", but it does this for both US and JP games. There is also an issue where the games I have that are supposedly the US versions are completely in Japanese(such as Magician Lord, even though the manual and box are english). Can someone explain this situation to me please? Is it a simple matter of changing the language preferences of the AES, or were some of these games not translated when they were released over here in the states? It's highly unlikely that someone packed a JP cart in the US casing and such but as I said this is unfamiliar territory for me and I don't want to misinform people when I sell these games. Thank you!

The carts contain the same ROMs for all regions. You have a Japanese BIOS in your Neo.

You have a japan bios so no matter what region the cart the system will play in japanese.

You can easily tell what carts are japanese by the print.

I see, thank you for the help.