Hi Bruno,
Form Builder just wants the value to be an NCName (validated with type="xs:NCName" in the XForms of the dialog itself, see dialog-control-details.xbl). This allows characters with accents, or Chinese characters for that matter, which is the idea, as we can imagine some form authors would prefer to use names written in their own non-English language.
I've tried to create a field named "prénom", I can then reference it from an XPath expression with $prénom, and didn't notice any issue. In the DOM, the id of the field with contain "prénom", which again shouldn't be a problem. Did you notice any specific issue when using non-ASCII characters in control names?
Alex
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