Well, I have to inform you that right now, Toolset Forms does not connect Parent Post when creating new Child Posts and WPML Core is used to translate the posts, Create Form unable to create new Post if WPML is active and O2M relationship is set
The issues require at least 2 languages, 2 CPT's (Custom Post Types), made translatable.
Those, if in a O2M (One To Many Relationship), will not connect the parent you choose in the form, if you create new child posts with Toolset Forms.
==> Disable WPML Core, issue dissolved.
So, pretty much, right now this does not work.
We ar working on the issues and resolving them and I can update you here in regard if you want once it's solved.
Related to your questions:
1. id of parenthighschool depends on language I use. So, for ru version ID=1, for de version ID=2, etc... So, I'm not sure how relationships tracks multilingual parent id. Probably I should forward some language independent post id for highschool to studycourse cred form?
Previously, in Types before 3.0 you had to set a hidden custom field value of the child post which saved the parent post ID.
That is not anymore like this.
We now have a new API and create database tables for the connections, so we can handle as well many to many relations.
There is however no API for the relationship forms yet that you can apply - there is only the Types API now:
https://toolset.com/documentation/customizing-sites-using-php/post-relationships-api/
This can help you set a parent, but not in a Toolset Form with API code as those new forms do not respond to that API.
When you want to preset a parent in a Child Form, it depends what kind of relation ship this is, I'd need to know more details about it so I can tell how to populate (if possible) the form to automatically connect a given parent.
In theory, as it will not work, as above outlined, as long WPML is active.
To the second question, you need to always first connect the default language:
https://toolset.com/documentation/translating-sites-built-with-toolset/translating-related-content/
In turn, you do not need to maintain connections between translations.
I hope the developers will have a solution for the issues as soon as possible so I can let you know here and you can proceed with the project.