Return all supported language pairs #68
Description
Developers often need to populate UI to, for example, let the user pick from any of the supported language pairs. Currently, this requires probing one by one if a desired language pair is supported. It'd be more ergonomic if there was one function to get all pairs. (Note that this function should not include data regarding each pair's availability to avoid fingerprinting, purely whether the API is capable at all of translating a given pair.)