For a client of ours we are using the LanguageManager to translate pages and content. The setup was easy and everything works.

The client is working in a specific businessarea and the translation should take that into account. In azure it is possible to add a category for that businessarea with a custom model, but in Optimizely there is no possiblity to add the custom model

How would we be able to do such a thing?

https://learn.microsoft.com/en-us/azure/ai-services/translator/custom-translator/how-to/translate-with-custom-model