You have a bit of a chicken-and-egg problem there.
Before you launch, few people will hear about you and even fewer will contribute content. But without content, your app isn't attractive enough. And the people it's most likely to attract are English learners, not English speakers.
I think you could somewhat get yourself out of this by becoming a multi-sided marketplace. E.g. Chinese users who want to learn English upload videos in Chinese, which unlocks English content for them. Those videos then get used on a site for people who want to learn Chinese. (hotpotchinese.com is already taken, but the maximally generic hotpotlanguages.com apparently isn't.) That way, network effects are in your favor.
It won't get you across the initial hurdle of not having enough content to launch at all, but you can try to get the flywheel going by providing content in your native language. I think that'll be faster than waiting for people to participate in this contest. (But maybe I'm wrong about that. How many videos did you get so far?)
To me, the way to get content is to pay for it. If there is not money for that, the business is under-capitalized (most are). If it doesn't make economic sense to pay for content, then the business itself probably isn't economically viable...it simply cannot achieve the imagined state where there is a market on which to charge rents.
