MediaWiki is fun. There are many, many Wikis and they are all different. Yes, we can create tests for a plain vanilla WMF Wiki. In reality they are all different. They are different because most have their specific modifications, their own gadgets. All have the potential to fail what passed the integration tests for plain vanilla wikis.
To known this, you have to have tests in the first place and the WMF Localication team is getting to grips with the testing platforms and building the tests that enable continuous integration.
Building tests is fun. You start with software that seems to be working perfectly and the tests prove that it has issues. This may sound odd, because it is not as if the software was not tested. A test that can be automated is validated in different ways. When a browser shows characters only a little bit different when comparing output, an automated test will catch this when it compares it to what is defined as correct. In many ways, automated tests test on a lower level then what humans do.
It is just that we do not have the humans to test over and over again.