[e-translate]: GET language pairs

[ghsnd]

A handy feature for a future release: possibility to get the language pairs by sending a (GET) request. This allows tools (e.g., our CKEditor plugin ;) ) to automatically get the possible language pairs, which has to be hardcoded at this moment.

[jnehring]

@andish does an Tilde API endpoint exist that sends the list of language pairs?

[andish]

Not yet. but I will create it, after review. It will serve a list of available translation systems: source Lang, target Lang, domain, system id.

[andish]

Duplicate of freme-project/e-services#20