I remember that question (I agreed as well). For some reason, in this case, I'd prefer the singular, though.
Despite the branch/branches example, it is highly unlikely that you're a manager for "one" international project, unless it's one over 30+ years and it's never THE international project, if you know what I'm getting at.
In contrast, European Project Manager could actually be misunderstood to mean someone who's working on the "European project" (and it does exist:
https://www.bbc.com/news/world-europe-34940241 ).
I should note, though, that a Sales Manager is always plural, whether it's AmE or BrE, but it typically is a Purchas
ing Manager. I think some of it you'll just have to learn by heart or decide on your own or do you disagree?
Best wishes