As adjectives the difference between frankish and merovingian

is that frankish is referring to the franks while merovingian is of or relating to this dynasty.

As a proper noun frankish

is the language of the franks, an extinct west germanic language.

As a noun merovingian is

one of a salian frankish dynasty that came to rule the franks in a region (known as francia in latin) largely corresponding to ancient gaul from the middle of the 5th century, and whose politics involved frequent civil warfare among branches of the family.