Ask HN: Is long-term use of an unpopular programming language bad?

Say I worked for several years building cool stuff at a job, but only mostly used F#, OCaml, Clojure or something. Will future employers think I would be a 'bad hire' because I haven't been coding in more standard languages like C# and Java that they might be using? I ask because I often see job postings that don't say much about software engineering skills in general but more like '5+ years developing production C# apps'. What if someone is a great engineer but happened to work with a different language?