Open-source / international community type projects
The common language of open-source and international community projects is English. Case in point, the language of the Stack Exchange sites is English. For broadest accessibility, English should be used for all code and object names.
Most international software firms mandate code to be written in English. It's the greatest common denominator, so it makes sense to use English as a means to create consistency.
Many regional software firms write in their native language. Not all follow this approach, but it makes sense from their point of view - they use the common language for all of their developers.
Naming business objects
The coding language of the team should be used to name objects.
The priority is for the developers to be able to communicate with each other regarding the objects and the project requirements. If the team writes in French, then the business objects should be named in French. If they code in English, then name the objects in English.
Your question adds a wrinkle because the client speaks a native language other than your team's coding language. You should still use your team's coding language to name the objects.
When the business analysts or developers need to communicate with the client about the specific business objects, an as-needed translation of the object name from the coding-language to the client language may be required. From my experience, it's pretty rare that I've talked about specific code objects with the client so the translation back to the client language hasn't really been needed.
The only exception to the naming rule is when an object has no other sufficiently concise term to capture the intent of the object. IMO, this is really rare, but it can occur. In reality though, it's the same thing spoken languages do to express particular concepts. "Facade" is originally a French term but has been adapted by English in order to express the concept, and is a common design pattern. "Schadenfreude" is another good example of a borrowed term although I don't think it has a corresponding pattern.