I assume you have already checked the user manual. Also, consider searching this forum site for various questions.
- It can be installed on Raspberry PI?
Yes. For example, OpenEMR successfully installed in raspberry pi 3 with php7(!) - works like a charme - Does it support 2fa?
Yes. - Does it have Lithuanian language translation?
The user manual does. However, the software is not available in Lithuanian. However, you can help create one if you are interested. - Client portal - can be installed on a subdomain?
This is open-source software, and, thus, pretty much anything is possible. However, I would only recommend this for someone with considerable web server software development experience and experience with OpenEMR – mistakes in this area could lead to introducing security flaws.
Enjoy!
–RBL