Hi Étienne,
Thanks for raising a new idea to help improve SDL TMS. Translation memories are primarily used in a single language direction only which is why there is only a one-way index create for each language pair. Creating a secondary index to reverse the language direction has a direct impact on the performance of the TM due to the overhead of keeping that index up-to-date with each new segments that is added. If the target language index is required to maintain the TM, I would recommend that you add it using the TM Maintenance tool and then remove it once the maintenance is complete. There are no plans to create the reverse index in SDL TMS. As we converge on a new cloud-based translation memory technology, it's possible that searching for text in the target segments becomes easier.
Regards,
David