when I go to the admin url like (mydomain.com/admin) I get 500 Internal Server Error but it just happened when I switch default language to something other than en_US when I change it to en_US everything is good and no error appended.
This is because static content is not compiled for this language, and no previous user had this language selected.
When compiling static, Magento uses only languages that are required, and in order to determine this, it looks into database for used languages. Since there were none other prior to your change, only English was compiled.
Easiest way to fix this is to run
bin/magento deploy:mode:set production after switching the language, though
bin/magento setup:static-content:deploy may be enough as well.