DaF; Deutsch als Zweitsprache; German as a second language; DaZ; German as a foreign language ; Critical period (language acquisition) ; Alter (Einfluss auf Spracherwerb) ; Age (influence on language acquisition) ; Age of acquisition; AoA; Sensitive period (language acquisition) ; Case study (second language acquisition) ; Fossilized interlanguage; Interlanguage (fossilized) ; Fossilization ; Transfer (second language acquisition) ; Transfer (Zweitspracherwerb) ; Cross-linguistic influence (second language acquisition) ; Interference (second language acquisition) ; Critical period (second language acquisition) ; Alter (Einfluss auf Zweitspracherwerb) ; Age (influence on second language acquisition) ; Ultimate attainment (second language acquisition) ; Near-native competence (second language acquisition) ; AoA; Age of acquisition (second language acquisition) ; Sensitive period (second language acquisition) ; Conjugation; Verb inflection ; Wortstellung; Satzfolge; Serialisierung; Serialization; Word order ; German ; Russian