Νομίζω ότι όλοι θα πρέπει να μαθαίνουν μια άλλη γλώσσα.

English Translation

I think everybody should learn another language.

I think this sentence is very bad Greek. It has been corrected on tatoeba to Νομίζω ότι όλοι πρέπει να μάθουν μια άλλη γλώσσα, which seems correct to me.