> I have intervention and control group and 3 measurement points. So, I would need to have six questionnaires that require different links.
Do you assign respondents to the groups within the questionnaire, or do you do that a-priori?
> I am using the table already. I was just not able to import this from another project and, thus, had to copy and paste every single item.
The language translation only works, if SoSci Survey can use a fixed base language as reference. If you have no common base language with identical terms in different projects, your table import will fail. But, as I wrote, usually you will do this within one project.
> So, I would need to have six questionnaires that require different links.
You can create multiple questionnaires (using the same questions) within one project, or you can use a single questionnaire, and use diffferent links to modify what is shown. And for experiments, you may only use one questionnaire, and automatically randomize the content.