For example:

Dutch is my mother tongue and I would like to learn English and French at the same time. I would have a vocabulary deck Dutch-English and Dutch-French.
If I would enter the word 'chair' in these decks, I would have two cards in two different decks: 'stoel-chair' and 'stoel-chaise' respectively.

But instead of learning only with my mother tongue as the base, I would like to learn these languages interchangeably (NL-FR, NL-EN and EN-FR).

In order to not have to add a third deck English-French with the card 'chair-chaise', it would be very convenient when Mnemosyne would recognise that 'stoel-chaise' contains the same word as 'stoel-chair' and asks whether you would like to link these cards for review, as they seem to have the same meaning.
This way you would keep them in seperate decks, but you would have the option to get the question in either three of the languages and be prompted to answer in either three.

The more language decks you would add, the more useful this would be.