Are computer languages copyrighted or have some restrictions imposed on them how they can be used? What does that mean in practice? If so, what can be done or what cannot be done? Could I make a compiler or ide or anything for any of them?
For example for Pl/Sql?
Unfortunately, programming languages may be encumbered by patents. This appears to be the case e.g. with the Aikido language.
Just recently this seems to have become a non-issue for the C# programming language (and the .NET Common Language Infrastructure).
To answer your question regarding what can and what cannot be done: if in your implementation of the language you use an invention that somebody patented, you definitely don't want to try to make profit with your implementation in any country where the patent applies (unless you licence the tech, of course). However, if you can circumvent the patent, i.e., implement for example a compiler for the same language without using that specific trick but something else, then you should not have a problem. Patents need (well, should need) to be very specific, so this might often be possible. (IANAL, though.)
You really need to familiarize yourself with copyright. Copyright applies to works of art: writings, paintings, etc. So the programming language itself cannot be copyrighted. The text describing it usually is, but that only prevents you from copying that text - it doesn't prevent you from reading it, understanding it, and using it.
So for PL/SQL, it's probably the case that its description is copyrighted by Oracle, but that can't stop you from making compilers and IDEs. As Pukku points out: there are other kinds of intellectual property, such as patents and trade marks, which may prevent you from doing these things (or calling them PL/SQL when done), but not copyright.