You can use $replace like this:
$replace(%FIELDNAME_WITH_SPECIAL_CHARS%,á,a,é,e,í,i,ó,o,ú,u,Á,A,É,E,Í,I,Ó,O,Ú,U,ü,u,Ü,U,ñ,n,Ñ,N,è,e,ê,e,ì,i,î,i,ï,i,à,a,â,a,ò,o,ô,o,ù,u,û,u,ç,c,Ç,C,ß,ss,œ,oe,Œ,OE)
From the documentation about "multiple parameter pairs y and z":
