English as a second language; ESL; TESOL ; Glottodidaktik; Lehrforschung; Pädagogische Linguistik; Pedagogical linguistics; Language teaching research ; Validity (method.) ; Evaluation criterion (method.) ; Classroom communication; Instructional communication ; Second language acquisition (pragm.) ; Zweitspracherwerb (pragm.) ; Interactionist hypothesis (second language acquisition) ; Conversational analysis (second language acquisition) ; English