My friend Colin Broughton completed the first full implementation of the Algo68 spec. Apparently there was some concern that it was not going to be implementable!
Reading the ALGOL 68 spec[1] today, it feels like complexity is mainly about the unusual verbiage used (which, as I understand, was equally unusual at the time, being introduced specifically for it). The language itself is pretty mundane by modern standards.
[1] http://www.math.bas.bg/bantchev/place/algol68/a68rr.html