Why is he considered a German actor when he's also Spanish?

He doesn't seem to have a problem with that. See his "Personal Quote" on his IMDB biography page, it starts with "We Germans ..."

He's the third Sperman I know of that are ok with their citizenships and lineages.

Fassbender is labeled as an Irish actor actually, not German.

Maybe he doesn't have Spanish citizenship? Christoph Waltz was born in Vienna yet only received German citizenship at birth through his German father. Laws on citizenship vary from country to country, not all of them have such a strong focus on place of birth like the USA.

when u spend ur childhood in germany

ur german

*beep*

Yeah, Daniel (a typical german name) grew up in Germany, spent his whole life in Germany, is well known in Germany, has had most Jobs in German cinema and his first language is german. Sure, he was born in Spain and genetically he may be Spanish and Brazilian, but (assuming you are from the US) would you consider yourself Mexican if you were born in Mexico and then moved to the USA after like 3 months where you would then spend the next 20 years of your life?

For many years it was common thinking that Mel Gibson was australian. He is in fact full born american, alltough he spent most of his younger years in Australia, to the point of having that characteristic accent.

I see where you're coming from but it is probably because he got famous in Germany with the movie Schule (amongst others) and generally people know him from those first few movies. Fassbender to me is an Irish Actor but solely because he grew up in Ireland and is like myself half-german, half-irish. I'm not sure if he identifies himself as german though..