English as a second language; ESL; TESOL ; Textbook analysis ; Linguistic imperialism; Globalisierung; Globalization; Transnationalization; Colonialism/Globalization ; Multikulturalität; Diversität, soziale; Vielfalt, kulturelle; Diversity, cultural; Multiculturalism; Transculturalism ; Periphery (linguistic geography) ; Core/periphery (linguistic geography) ; Three circles model (of world Englishes) ; Center/periphery (linguistic geography) ; English as international medium ; English