No, if you do that the sentence will become ungrammatical: * To identify themselves as American citizens, they must use the English language not too difficult to understand. You must link the AdjP "not too difficult to understand" to the NP it's modifying. To do that requires a relative clause using the relative pronoun "which" (or the subordinator "that") as the relativised word linked to the NP antecedent "English language": To identify themselves as American citizens, they must use the English language [which/that is not too difficult to understand ] .
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