A few days ago, I decided to submit a paper to a journal ran by de Gruyter. They have their LaTeX package, but it turned out it didn’t work – even with their own document template. The error message was
Package babel Error: You haven't defined the language ngerman yet.
It turned out that what needs to be done (at least in TeXlive) is to install the hyphen-german package.
Go figure.
(Admittedly, if you and all your testers run a TeX installation geared toward German users, that might be tricky to even find out. OTOH, is it really possible that I was the first one to encounter this issue, especially that the package has been out for almost a year now?)