>So, because en contributors changed their description, you, yourself, alone, decided to change the label
That's why they call it wiki.
>in a language that is not yours, without even discussing it with the concerned language community...
Wikidata is a multi-language project. A property has to convey the same meaning through different languages; I know some French and our languages are very similar; I perfectly know the difference between 'affirmé dans' and 'source of déclaration', they translate literally in Italian.
>do you really think your understanding of French is better than ours?
So in French is it correct to say 'affirmé dans': 'Polydore Vergil'? That's the example usage reported in the property page, not something I invented on my own.
>fr description already is explicit about the "source only" use
That is the problem. The property is not only about works, but any other information source, too. Simply 'affirmé dans' is not correct when you are using humans, organization or other types of items for your statements.
>Please let french contributors discuss and decide the modification of fr-labels if they think it's necessary... do NOT do this without discussion in the proper place
Again, if your community decides to ignore how the property works, I do not have to discuss it in your language's project page. At max we can open a section in the discussion page if you think I am wrong, but again, I simply replaced the label to reflect property usage.