Summary
Under service Project Settings >> Language Support, there's the following description:
Translate your customer portal and email notifications into multiple languages for your customers. If their preferred language is not available, they will see the default language you select.
This causes some confusion as previously reported in the following Bug ticket (closed as not a bug):
Suggestion
As per the notes in the bug closure, the behavior is not clearly explained in the text and could receive some adjustments for better understanding:
When you set a default language, you are telling JSM that you have setup the field information in JSM settings with the default language.
Meaning if you set French as the default language, you are expected to translate all the field information in every request type to French.