When rolling out b2b SaaS software internationally, where should language preference be set? At the customer/business level, or at the user level? Example: I onboard a customer group in the Netherlands to my SaaS product. The customer generally speaks Dutch and English. Should I build my application so that all users of this customer get the website rendered in Dutch, or should I let each individual user choose based on their browser/system/personal preferences?

Always let the user choose. The Netherlands' example is a good example of why choice is important. Many people in the Netherlands do not speak Dutch, and it's very frustrating to have to look at websites in the wrong language.

I would try to make the language as user-specific as possible. Often, users have ways to set their preferred language. If you know their language preference, you should use it. You can find this out by asking them in the UI, obtaining their system info if you are using an app, or through HTTP headers like Accept-Language. If they aren't logged in or don't have a cookie set, I would defer to demographics. In the case you mentioned, I would default to Dutch.