Before revision:  One particularly challenging phenomenon is the use of multiple languages within one utterance, also called Code-Switching (CS) . Whereas monolingual social media data already provides many problems for natural language processing, CS adds another challenging dimension.
After revision:  One particularly challenging phenomenon is the use of multiple languages within one utterance, also called code-switching (CS) . Whereas monolingual social media data already provides many problems for natural language processing, CS adds another challenging dimension.