When writing config.yaml keep translation handling in mind.
After adding a config.yaml to a client's existing website we suddenly experienced mixed languages for content elements. As it turned out there were not always originals defined in the translated content elements. This was the reason why the language overlay fell back to displaying the content element in the original language.
Aside from establishing relations from all translated content elements to their original (in default language) we solved this problem by changing the fallbackType in config.yaml:
instead of strict or fallback.
Mind the translation handling.
When adding a config.yaml to an existing project - check how the editors used translation handling and check the pages wether there are language inconsistencies.