So, which language should they use? Java? C#? Knuth's MIX? What if the job requires a different language -- does the Engineer's professional certification sud

So, which language should they use? Java? C#? Knuth's MIX? What if the job requires a different language -- does the Engineer's professional certification suddenly become invalid, thus denying him work because of a technicality, even though he otherwise has tons of experience in that language? Also - that would really fragment the hiring scene. But a bonanza for the testing company.