I don't know about European countries, but the definition of treason in the US is very narrow. "Treason against the United States, shall consist only in levying War against them, or in adhering to their Enemies, giving them Aid and Comfort. No Person shall be convicted of Treason unless on the Testimony of two Witnesses to the same overt Act, or on Confession in open Court.". So unless things escalate into an open US-Wakanda war, working for Wakanda can not be considered treason, and even then you would have to prove it in Court.