Thank you for your response.
I have actually created the termbase myself using Glossary Converter just like the video.
The problem is that now we want to make some changes in the terms directly in Multiterm and it's doing weird things, like what I said in the main post, when we try to merge certain terms, some of the terms from the same entry from other languages disappear.
Yesterday, instead of merging the terms, Multiterm was actually duplicating them.
We have tried uninstalling Multiterm and installing different versions, but none of them work as we would like.
When I have time, I'm going to try to create the glossary again from scratch in a different way, just to see if it works for some reason.
Thank you anyways :).