All ASN.1 character string types, including multi-byte types such as BMPString and UniversalString, are mapped to the Go string type. The Go string will be interpreted by the generated code as UTF-8.
The useful character string types in ASN.1 are as follows:
UTF8String ::= [UNIVERSAL 12] IMPLICIT OCTET STRING NumericString ::= [UNIVERSAL 18] IMPLICIT IA5String PrintableString ::= [UNIVERSAL 19] IMPLICIT IA5String T61String ::= [UNIVERSAL 20] IMPLICIT OCTET STRING VideotexString ::= [UNIVERSAL 21] IMPLICIT OCTET STRING IA5String ::= [UNIVERSAL 22] IMPLICIT OCTET STRING UTCTime ::= [UNIVERSAL 23] IMPLICIT GeneralizedTime GeneralizedTime ::= [UNIVERSAL 24] IMPLICIT IA5String GraphicString ::= [UNIVERSAL 25] IMPLICIT OCTET STRING VisibleString ::= [UNIVERSAL 26] IMPLICIT OCTET STRING GeneralString ::= [UNIVERSAL 27] IMPLICIT OCTET STRING UniversalString ::= [UNIVERSAL 28] IMPLICIT OCTET STRING BMPString ::= [UNIVERSAL 30] IMPLICIT OCTET STRING ObjectDescriptor ::= [UNIVERSAL 7] IMPLICIT GraphicString