Currently, there's no way of translating message texts (used in show_message
etc) used in trigger code.

Proposal:
Introduce a <message name="msg_foo" text="You entered %(bar)s, and that's
foo!"/> tag that can be used in translation gfd's. Change show_message in
trigger namespace to accept something like show_messagE(msg_foo,
bar=fld_bar.value, kind="Error")