When the locale is set to e.g. iso-8859-1, gpg 2.0 will send a string like "Sie
benötigen eine Passphrase" (de_DE) in this encoding to the agent which in turn
transmits it to pinentry. If the string contains utf-8 encoded information from
a key, pinentry will get confused.
Gpg 1.4 /always/ used utf-8 for all strings sent to the agent/pinentry, even if
the locale was set differently. GnuPG 2.0 should also follow this approach.