On CX2 testing server, it seems to be impossible to start translation to languages using Yandex - e.g. from English to Russian. Click on 'Add translation' will display the loading icon (the load never finishes) and the warning regarding http://cxserver2.wmflabs.org/v2/translate/en/ru/Yandex:
Failed to load resource: the server responded with a status of 403 (Forbidden)
[CX] Unable to get cxserver token (ignored). {error: {…}} mw.cx.MachineTranslationService.fetchCXServerToken @ VM241:26 mw.cx.MachineTranslationService.getCXServerToken @ VM241:26 mw.cx.MachineTranslationService.translate