How to change mc (midnight command) bash prompt on OS X?
The issue is that .bash_login files are only read when bash is run as the login shell. mc probably just calls a new shell which will read its startup from ~/.bashrc See bash documentation for details.

To make the shell run in a common way it is often suggested that you call .bashrc from .bash_login e.g.

if [ -f ~/.bashrc ]; then
   source ~/.bashrc
fi