C would seem like a good guess: C is popular for embedded systems, satellites are embedded systems, therefore C is popular for satellites. There are a lot of holes you can poke in this line of reasoning, and of course many languages have been used to varying degrees of popularity. In the absence of a survey of languages used that would answer this question decisively, the examples here can give some context.
Keep in mind that since a significant number of satellites do not have publicly available information, such as military and spy satellites. One could guess that Ada may be more popular than a general survey would suggest, given it's widespread use in the US military in the past. More information on Ada is provided in this related question.
A specific example of C's use: NASA's Core Flight System (cFS homepage) is written in C, and is in use on several missions right now (related question).
Smaller platforms such as CubeSats may change the languages in space distribution as well: there are many of them and they have a much higher risk tolerance (and lower success rate). From the LightSail paper presented at SmallSat 2015, "LightSail FSW (software and firmware) is written in the
C programming language." LightSail also ran a Linux distribution, again written in C.
Assembly was also a popular language, at least historically, according to Ron Garret who wrote the following about working at JPL in 1988: "Spacecraft were mostly programmed in assembler, or, if you were really being radical, Ada." The article with this quote documents the work done on Remote Agent, an experimental LISP program that controlled the Deep Space 1 satellite for two days during its mission.