One approach is quite simple:
Ask the user to translate a piece of code (such as a function/method/class) from [language A] to [language B], where language A and B are listed in his list of "known" languages.
With proper language-agnostic unit tests, and an environment that can execute most of the popular languages, you can easily test if the code is valid or not.
For example, you could manually provide the source code for FizzBuzz in Javascript. Then, someone that knows Javascript and C# would translate the FizzBuzz to C# (where the definition of the FizzBuzz problem doesn't have to be written in word, as it's already there in code). Then, if the tests pass, the service now has FizzBuzz in 2 languages. Any of these 2 can be used as input for translation/porting for other users, until the algorithm/code is covered by most major programming languages.
You could easily expand on this by either providing new algorithms/code, asking users to tweak the code a little to generate a new unique algorithm, or even generate random algorithm.
This is not a replacement for proper tests and interviews, but would probably be much better than anything out there to quickly test if someone is making false claims on their resume or not.
TL;DR: Ask people to translate an algorithm from a language they can read to a language they can write. Use their answer as input for other users, for minimal manual content creation, and covering any language your service can run and unit-test.