Scientists predict that Language X will compete with English in global usage.
Another expression or word instead of "compete with" in this context?
How about vie?
vie, verb (used without object), vied, vying.
- to strive in competition or rivalry with another; contend for superiority
A phrase that is often used in situations like this is vie for dominance (Ngram). You could say:
Scientists predict that Language X will vie for dominance with English in terms of global usage.
How about rival?
Having the same pretensions or claims: competing
1 : to be in competition with