Thanks for raising those points. We are aware of the community concerns about the use of external proprietary services.
We have shown that Open Source translation engines are our big priority by integrating Apertium already. Unfortunately, Apertium supports about 36 language pairs, while other services such as those provided by Microsoft or Yandex support about 1936 language pairs (all possible combinations of 44 languages).
Increasing the language coverage is important to help users to expand the sum of all human knowledge using Content Translation. The current patchset is intended to test the use of multiple services and it is using Yandex just as a convenient example. This does not mean that integrating that specific service is in our immediate plans.
It is worth noting that in any case we are not including closed source software in our codebase. The interaction with external services is based on sending the existing content of an article (not including information about the user or any other user input). This is not very different from other examples such as location template linking to Google maps in addition to OpenStreetmap. In any case we are in conversations with the Legal department to make sure there are no problems in that regard.
Beyond technical and legal issues, we won’t be deciding in the name of the communities. We think it is our duty to prepare a software so that it can help users from as many languages as possible by accessing to as many services as we can (and we welcome the community to integrate all kinds of services in our platform), but each community will have their say and would be able to configure which services are made available. Even more, individual users will be able to select their preferred translation service from those offered (and we’ll keep any Open Source option available as the default).