I didn't find any useful information about programming languages for real time systems. All I found was Real Time Systems and Programming Languages: Ada 95, Real-Time Java and Real-Time C/POSIX (some pdf here), which seems to talk about extensions of Java and C for real times systems (I don't have the book to read). Also, the book was published in 2001, and the information may be obsolete now.
So, I'm dubious whether these languages are used in real world applications, or if real time systems in the real world are made in other languages, like DSLs.
If the second option is true for you, what are the outstanding characteristics of the language you use?
The Real-Time Specification for Java now has several commercial-grade implementations:
These products span the continuum from compilation to native code (Aonix) to J2ME (aicas, apogee), to full J2SE (Sun, IBM). Most, if not all, have seen deployments in small numbers of safety- or mission-critical systems, but momentum is building. Examples include Eglin AFB's space surveillance radar modernization and the US Navy's use of RTSJ in the DDG-1000/Zumwalt destroyer. Sun also claims deployment in the financial transaction processing domain.
If you are interested in RTSJ, I suggest Peter Dibble's Real-Time Platform Programming, or Professor Wellings' Concurrent and Real-Time Programming in Java.
On a related note, there is also work underway to provide a Safety-Critical profile for the Java programming language, built as a subset of RTSJ. Also, an expert group has formed to explore a Distributed RTSJ DRTSJ, but the work is stalled.