Is the GBA SP region locked?

No, it isn't

As I know there is no restrictions of region in Game Boy Advance games. I own European console and 2 USA games witch work great.

Nope

Well I played the Japanese version of Emerald green when it came out. It had Japanese language and everything and it worked fine for me. Do they region lock GBA games?

E: BTW I'm not Japanese or anywhere close to Japan.

E2: Well it's written in the description

"These games are not Uk English version but are playable in English and will work on Uk GBA consoles"