Last modified: 2013-08-19 15:12:39 UTC
The issue is that on Russian Wikipedia API works only with the JSON format, using all others simply fails with <error code="unknown_format" info="Unrecognized value for parameter 'format': xml"/> (that's what I originally used), but if you try to change to any other it will return the error message in the format specified. Using Special:APISandbox produces the same behaviour.
Disregard, after further testing it happens only w/ the templatedata action, so might be intended.