Case study (language acquisition) ; Differenzierung (Zweisprachiger Spracherwerb) ; Differentiation (bilingual language acquisition) ; Language differentiation (bilingual language acquisition) ; Second language acquisition (psycholing.) ; Case study (second language acquisition) ; Zweitspracherwerb (lex.) ; Second language acquisition (lex.) ; Vocabulary (second language acquisition) ; Second language acquisition in children; Kindlicher Zweitspracherwerb; Child second language acquisition ; Word classes ; Declension ; Number ; Conjugation; Verb inflection ; Questions; Interrogative sentence ; Dependent clause; Subordinate clause ; Wortstellung; Satzfolge; Serialisierung; Serialization; Word order ; German ; Russian