field labels postfixed with context (po msgctxt) beetween []

Hi all,

I have a D7 site set up with multilingual support (en & fr actually) and comments (on Articles).
Everything goes fine, except for field labels which are not part of the 'Built-in interface' text group: when displaying a page in French containing such a field, its label is displayed postfixed with the context (msgctxt from .po file) enclosed between square brackets.

For instance, when dispaying the page (in French) for an article node with comments allowed, the 'Comment' field label appears as

• Comment [fr:field:comment_body:comment_node_article:label]
• Commentaire [fr:field:comment_body:comment_node_article:label]

depending on whether this label is actually translated or not.

So First, I wonder why the the 'Comment' label is part of the 'Fields' text group and others (like 'Your name', 'Subject'...) are parts of the 'Built-in interface' text group' !?...
Second, I get confused when I read here that

The translation of the field label is provided by the core Locale module.

Whatever, I have the same issue with any other custom field I define on the 'User' entity ('First Name' for instance). So, this can not be a 'Comment' module issue. As well, I don't think it can be a 'Locale' module issue: I would otherwise experiment the same trouble for any form 'property' (or whatever they are named, since they don't seem to be 'fields'...).

What puzzles me is that it looks like nobody reported such an issue so far...