Multilingual speakers communicate in more than one language in daily life and on social media. In order to process or investigate multilingual communication, there is a need for language identification. This study compares the performance of human annotators with automatic ways of language identification on a multilingual (mainly German-Italian-English) social media corpus collected in South Tyrol, Italy. Our results indicate that humans and Natural Language Processing (NLP) systems follow their individual techniques to make a decision about multilingual text messages. This results in low agreement when different annotators or NLP systems execute the same task. In general, annotators agree with each other more than NLP systems. However, there is also variation in human agreement depending on the prior establishment of guidelines for the annotation task or not.