Hello, thank you for this question. In my opinion, it depends on the country you are in and the nationality of your learners. In Europe, it would generally make sense to use English. However, if you are in Spain, for instance, with Spanish and Italian learners, I don't think English would be a nice choice. You should probably ask students if they all feel confident with Spanish, because Spanish and Italian are quite similar. When I did not know any Spanish, I felt really stupid speaking English with them. So sometimes we either opted for their Spanish or my Italian and we could understand each other quite easily (false friends/cognates apart, as in every language of course)!
This scenario is mostly noticeable in my country DRC where we have students speaking different mother tongues or first languages. Since those classroom students speak various languages, English qualifies as a lingua franca bridging the gap of students' understanding. Congolese students are very smart that they can rapidly learn a new or foreign language. However, in monolingual classroom, English language might be a real barrier for students are not used to practicing more than one language.
Obvious, there is no barrier if your proficiency level is high. it becomes more of an obstacle if your English is weak. Your knowledge of the speciality is also important if you wish to communicate efficiently.
In multi-lingual classroom it is definitely a bridge to learning because the students do not all speak the same language and so have to use English to learn the subject matter and to communicate among themselves.
I remember that during an international conference in Jordan, we Arab linguists/researchers were discussing in English outside the conference rooms!!! So English was our lingua franca although we could use MSA between us.
University campuses in non-Anglophone countries are increasingly viewing English-Medium Instruction (EMI) as their passport to global academic relevance, driven by global competition, improved rankings and a larger share of the international student market...
"For many universities, English-Medium Instruction (EMI) is not just an option but a strategic imperative. It acts as a powerful magnet for attracting international students who might otherwise be deterred by language barriers, and it serves as a key domestic tool to prepare local graduates for a globalised workforce.
With more than five million students now studying abroad globally, EMI has been widely adopted in many non-Anglophone countries across Asia, Europe, South America and other regions as a strategy to simultaneously enhance students’ English language skills and deepen their subject knowledge.
Even non-English-speaking countries have used EMI in academic programmes at university-level including Ethiopia, Rwanda and Algeria, with the last two switching from French to English in higher education in recent years.
Several nations now host universities and branch campuses where English is the primary language of instruction, including entire graduate programmes..."