Second language acquisition (language teaching) ; Zweitspracherwerb (Unterricht) ; Foreign language teaching ; English as a second language; ESL; TESOL ; Wortschatz (Sprachunterricht) ; Lernwortschatz; Vocabulary teaching ; Lexical complexity measure; Wortschatzreichtum; Lexical sophistication; Lexical diversity; Type-token ratio (lex.)