Coding questions presumably belong on Stack Overflow; other questions can go to Webapps.SE.
Stack Overflow will naturally take programming questions relating to the service.
But it appears that Webapps.SE will take other Transifex questions; looking through their [transifex] tag, it appears that general questions about the service are allowed, such as Username change on Transifex and How do you do a Transifex text search with an “or” operator?. So, if you have a general question about Transifex, it should go to Webapps.SE; if you have a programming question relating to Transifex, it should go to Stack Overflow.