Both may be correct, as "language" may be countable or uncountable. The second one would be more likely, since the first implies that English as a language is made up principally of curses and swear words. The second one could refer to a particular utterance, or diatribe.
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MustAskan awful languageAwful languages may include English, Chinese, Swedish, French, and so on. Each of these is an example of a language.
MustAskawful languageAwful language may include ungrammatical language, vulgar language, incoherent language, and so on. Each of these is an example of language.