I can beta Spanish (I'm fluent -- a native speaker who still lives in a Spanish speaking country), particularly latinoamerican (Argentinian, but I can manage a small bit of slang and usual speech patterns of some other countries, as well, and check with the wide internet and some people I know if there are doubts). Culturally, I can only check for Argentina (Buenos Aires; south, think Bariloche; and north, think Formosa), though I could also ask specifics questions about Spain, Paraguay & Mexico to friends.

Doubt anyone would need it, but I'm here in any case.