This paper is based on research conducted in banks in the international banking centre of Luxembourg. Operating in the globalised context of international banking and multilingual Luxembourg, financial institutions are an ideal focal point for investigating policy and practices in complex multilingual and transnational work spaces. Two theoretical models are the primary frameworks used to explore top-down management (explicit official language policy) and employees' practices and beliefs (implicit policy). The database for the study includes a first phase of interviews with management in 10 banks, followed by a second phase of questionnaires and focus groups with employees in three case study international banks. Taking an exclusively bottom-up perspective, this paper explores employees' complex practices through genres of communication. An analysis of employees' broad frequency patterns indicates that English is the language most frequently used alongside others. A number of languages are flexibly used in meetings, informal communication and more hybrid genres (emails and presentations). The ethnolinguistic composition of employees and transactional/relational functions of language are the two most important bottom-up pressures on language choices. However, above all, English emerges as an essential lingua franca for involving and including all employees in these contexts.