Hello!
How it is possible to download a card of Lithuania in a Polish format (.MP)?
Thanks for your explanation. 
I’m not Lithuanian, but you should be able to download an OSM excerpt with Lithuania from download.geofabrik.de and then use osm2mp tool.
I personally never used any Garmin-related tools, but that’s what I know.
Fortunately, if you have any problems along the way, the Garmin maps sub-forum seems to be active:
https://forum.openstreetmap.org/viewforum.php?id=26
This service does not provides .MP format, but You can find many other popular formats of Lithuania(strict borders).
http://osm.ramuno.lt/