The answer was simple. I just needed to explicitly set CONTENT_TYPE:
from("direct:start").
setHeader(Exchange.HTTP_METHOD, constant('POST')).
setHeader(Exchange.CONTENT_TYPE, constant('application/x-www-form-urlencoded')). // this one did a trick
setHeader('X-HTTP-Method-Override', constant('GET')).
setBody(constant('q=Hello')).
log(LoggingLevel.INFO, 'sourcingtool', '${body}').
to("https://www.googleapis.com/language/translate/v2?key=${api_key}&target=fr").
to('stream:out')