The aim of this paper is to introduce, analyze, and discuss the Language Development Test (LDT) as a tool for examining bilingual children. The research was based on a case study of a boy who was at the stage of acquiring Polish as an L2. The obtained test result differs significantly from the opinion of the Polish language teacher and speech therapist who cooperated with the boy from the moment of his arrival in Poland to the time of the final examination. Alex’s score on the Language Development Test (TRJ) corresponds to the third level. In reality, however, the boy mastered the language well enough to begin his education in a Polish school, keeping up with his Polish peers in acquiring knowledge and expanding his linguistic competence. This demonstrates the lack of valid tools for examining bilingual and multilingual children. Standards for these groups should be developed separately.