Case study (language acquisition) ; Adoption (language contact) ; Cross-linguistic influence (psycholing.) ; Language loss (biling.) ; Attrition (biling.) ; Language attrition (biling.) ; Second language acquisition in children; Kindlicher Zweitspracherwerb; Child second language acquisition ; English ; Mandarin; Kantonesisch; Cantonese; Putonghua; Sinitic; Hakka; Yue; Chinese