Feature: Check if Language Weaver is compatible with only one MT API

[tovmasharrison]

Problem [optional]: Currently, for the Language Weaver quality estimation API in quality_estimation.json, the only_compatible_mt_api is given null (i.e. "only_compatible_mt_api": null), which means, it's compatible with any MT API.

Reference: #646 (comment)

Request: Confirm if this is actually true, or if it only works with a specific MT API, and modify it accordingly.