Natural Language Processing (NLP) is a crucial branch of artificial intelligence that examines how people and computers communicate through natural language. The majority of NLP research focuses on the fundamental methods that enable humans to understand and generate words, phrases, sentences, and the whole documents. The primary focus of the studies in this area concentrate on English as the common language used by scientists. Unfortunately, this language has a very straightforward structure. It indicates that most of the techniques that are very successful for documents in English do not work equally well for texts in other niche languages.
To investigate the existing studies on Natural Language Understanding (NLU), a Systematic Literature Review (SLR) was performed. Through our deep research on NLU and its extensions, 46 studies were selected mentioning 38 machine learning, 3 hybrid and 4 knowledge-based approaches to NLU.
The paper presents the systematized knowledge along with a critical analysis of existing research, both for text processing in English and for other native languages. It shows the solutions and reasoning of researchers, and compiles the advantages and disadvantages of the presented solutions.