Hello,

I see a problem in the Pages module. I have created relational headings to avoid using TAGs which simplifies the keywords but I realize that it is not possible to assign several times the page in relational mode.

Not sure I understand the issue here. This sounds like a setting issue. There is a setting to allow multiple. I know because I have relationship fields that relate multiple items:

I will make an example:

1. I created a database-pages (Mendeleiev) without categories with the terms of the Mendeleiev table.
2. In another database-pages (english) with caterogies, I have defined a field that is relational with the Mendeleiev database and work fine.
3. Now I want create and another database-pages (french) and use field that is relational with the Mendeleiev it's not possible because it was already used...

You have to create another database relationship field. You can't include multiple databases in a single field, this is intended. But you can still style it all the same.