How to convert base64 string to utf-8

Hello All,
I'm using utf-8 Postgres database, where I save strings in many languages.
I have to match the database with strings encoded in mime base64 or 
quoted-printable format. Like next:
=?utf-8?B?15TXoNeUINee16nXlNeZINeR16LXkdeo15nXqi4=?=
or
=?KOI8-R?Q?=F0=D2=C9=D7=C5=D4=2C_=ED=C9=D2!!!?=

I think that I need first convert these strings to utf-8, but I can not 
find out how to do it.
Thanks for any help,

Alexander N. Treyner.