translate: allow clients to ask for supported output formats

translate: allow clients to ask for supported output formats

Currently translate asserts on unsupported output formats, making
it impossible to use for some purposes, such as testing whether it
actually works on all formats it supports.

Removing the assert was met with opposition, so this change allows
clients to ask whether an output format is supported, and they are thus
able to avoid attempting to use it.

Since this is just an addition to the API, no adverse effect is
possible, and it makes the testsuite work again.