Mandarin vs Spanish, which should I learn???

I dunno which should I learn.

Spanish is a secound-language in USA,
Mandarin is most spoken in the world, because China is a big country.

Alot of people speak Spanish in the USA
Fewer people speak Mandarin in the USA

Spanish and chinese is more complex than English

Chinese is complex but uses a tonal language
Spanish is complex and dont ussa a tonal language


German used to be ammerica`s secound language , but something happened that they changed it to Spanish , so couldnt the same thing happen to Spanish?