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.
Considered stable (it is using production version, 6.0.0) although I haven’t tested the docker up for awhile on raspberry pi. We incorporated multi-architectural docker builds in the pipeline awhile ago for this specific purpose (as an aside, it increases the docker builds from 5 minutes to 90 minutes, so good to see it may actually be used ).