How can I use a non-translated referenced item in another language?

Quite cumbersome to give a proper title; here is the situation:

Content A+B are translatable.

Item B1-english references item A1-english.
Item A1-english has not being translated, hence there is no item A1-french.

I would like item B1-french to refer to item A1-english because there is no A1-french item to refer to.

Note that when editing B1-french I cannot refer manually to A1-english because I can refer only to French items (this is a good thing).

So how could I achieve this? Any help is appreciated, thanks!