Őőőőőőő, are you there?
Scala, 56 bytes

s=&gt;new java.awt.Font(s,0,0).canDisplayUpTo("ŐőŰű")&lt;0


canDisplayUpTo is a method of the Font object that returns the index of the first charater that cannot be displayed by the font. It return -1 if all characters can be displayed. javadoc