Is there a language w/out dependent clauses, i.e. one in which you couldn't say e.g. "I know that x is here" w/ a clause, where the main clause is in italic and dependent in bold (and that overlaps, because you may analyze it as belonging to this or that). I suppose the lack of dependent clauses might be possible if a language would exclusively use a different constituent (i.e. not a sentence or clause) to convey the same meaning. Or maybe you lack so complex syntax in some pidgins (which would be unlikely to generalize to full-blown natural languages)
NOTE: The original question was about "relative clauses", which seems to be a proper subset of dependent clauses that I intended it to be about