I enabled the series taxonomy in my multi-language blog. Maybe I don’t understand correctly how series work together with multi-language support. But how can I translate the series? If I add it in English to an English article and in German to a German article, both translations show up on the series page of each translation.
I am unable to reproduce the behavior you describe. Here’s a simple multilingual site to try:
git clone --single-branch -b hugo-forum-topic-39140 https://github.com/jmooring/hugo-testing hugo-forum-topic-39140
cd hugo-forum-topic-39140
hugo server