[Bundessprachenamt. (2008). Sprachprüfungen. Bundessprachenamt. Retrieved from http://www.bundessprachenamt.de/deutsch/produkte_und_dienstleistungen/sprachpruefungen/sprachpruefungen.htm]Search in Google Scholar
[Crystal, D. (2003). English as a global language. Cambridge. Cambridge University Press.10.1017/CBO9780511486999]Search in Google Scholar
[Definitions of language levels. (n.d.) Retrieved from www.englishschool.org.uk]Search in Google Scholar
[Graddol, D. (2000). The future of English? A guide to forecasting the popularity of the English language in the 21st Century. London: British Council. Retrieved from https://www.teachingenglish.org.uk/article/future-english]Search in Google Scholar
[Jenkins, J. (2005). ELF at the gate: The position of English as a lingua franca. Humanising Language Teaching, 7(2). Retrieved from http://www.hltmag.co.uk/mar05/idea.htm]Search in Google Scholar
[Jenkins, J., Cogo. A. & Dewey, M. (2011). Review of Developments in Research into English as a Lingua Franca. Language Teaching, 44(3), 281-315. doi: 10.1017/S026144481100011510.1017/S0261444811000115]Search in Google Scholar
[NATO. (1949). Final Communiqué of the First Session of the North Atlantic Council (Terms of Reference and Organisation). North Atlantic Treaty Organisation. Retrieved from http://www.nato.int/cps/en/natolive/official_texts_17117.htm]Search in Google Scholar
[NATO. (2014). STANAG 6001 Language Proficiency Levels. NATO Standardisation Office. Retrieved from http://nso.nato.int/nso/]Search in Google Scholar
[NATO. (2014). ATrainP-5. Language Proficiency Levels. NATO Standardisation Office. Retrieved from http://nso.nato.int/nso/]Search in Google Scholar
[MŠMT. (2013). Seznam standardizovaných jazykových zkoušek, č.j. MSMT-24195/2013. Věstník Ministerstva školství, mládeže a tělovýchovy, 9/2013. Retrieved from http://www.msmt.cz/dokumenty/2013-09]Search in Google Scholar
[Lingua franca. (2015) In Oxford Dictionaries. Retrieved from http://www.oxforddictionaries.com/definition/english/lingua-franca?q=lingua+franca]Search in Google Scholar
[Seidlhofer, B. (2004). Research perspectives on teaching English as a lingua franca. Annual Review of Applied Linguistics, 24, 209 - 239. doi: 10.1017/S026719050400014510.1017/S0267190504000145]Search in Google Scholar
[Seidlhofer, B. (2011). Understanding English as a lingua franca. Oxford: Oxford University Press.]Search in Google Scholar