The notion of language proficiency in English Language Teaching (ELT), as an internationalized educational enterprise, has tended to be operationalized in terms of stable lexicogrammar and enduring normative patterns of social use. It will be argued that this 'established' stability has been challenged by the scholarship in several fields of applied language studies that has demonstrated the 'leaky' and mutable language boundaries and emergent communicative enactments in situated use. The main theme of this article is on the coalescing professional and curricular recognition of the fact that use of English is contextually and functionally variegated, and can include resources from other languages. I will draw on relevant works from research fields such as academic literacies, English as a Lingua Franca, flexible multilingualism, and translanguaging to illustrate this growing understanding. Some aspects of the Common European Framework of Reference for Languages (CEFR) will be discussed as a curricular example of the movement towards a more linguistically fluid and interactionally accomplished notion of language proficiency. In the final part of the discussion I will look at the key challenges brought about by this more contingent and situated view of language proficiency/ies in terms of pedagogy and assessment, and some of the potentially productive directions of investigation for further development.