But you don't know sarcasm.

BuT yOu dOn't KnOw SaRcAsM.

Why would the spell work for translating sign language? For that matter: Could sign language even plausibly develop in the Forgotten Realms when the cure for deafness is one Lesser Restoration away?

Quote from SootyForge >>

Why would the spell work for translating sign language? 

Because it's magic. Why would sign languages be any different?

Quote from SootyForge >>

For that matter: Could sign language even plausibly develop in the Forgotten Realms when the cure for deafness is one Lesser Restoration away?

Yes. Most people probably don't have easy access to spells like that, and sign languages are frequently used by people who aren't deaf.

Clearly the caster is a slow reader.

And Common Sign Language should be included along with various class cants.

We just used sign language in the game we're playing to speak between ships that were too far away for voices. In fact, that's why we gave the characters sign language.

I've searched the spells and found nothing that allows you to SPEAK the other language. Does understand mean you can speak it? It doesn't seem to say that. It says you can understand it if you hear or see it.

Quote from BradBeyond >>

I've searched the spells and found nothing that allows you to SPEAK the other language. Does understand mean you can speak it? It doesn't seem to say that. It says you can understand it if you hear or see it.

No, this spell does not give you the ability to speak languages you don't know. It only allows you to understand the literal meaning of them when you hear or see them used.

Spells only do what they say they do.

The third-level spell Tongues does allow you to make yourself understood by people who don't normally speak your language(s).

There we go. Thanks.