The native culture is always used as the base for translations since it’s the only source text we’re guaranteed to have available to translate from. If your native culture is set to French then that means that French will be used as the basis for all your translations and there’s no way to change that behaviour without changing your native culture (which would involve updating all your source text).
Another idea would be to add the English text as a comment on each PO file entry so that the translators would be able to see the current English translation (if available) as well as the French source text.
This isn’t currently possible (unless you implemented it yourself by changing the PO export code), but I could put in a feature request for 4.14/4.15.