> Besides, as a professional (computational) linguist, I have met many trained/professional linguists that know only one language - typically English - well - I think that is professionally totally unacceptable when your mission is to study language (in abstract, intentionally without article here => linguistics); the way a language structures the world for you in the way its vocab influences the way you think in concepts (Sapir-Whorf hypothesis, AKA linguistic relativity => https://en.wikipedia.org/wiki/Linguistic_relativity ) is something that you should at least occasionally step out of, so that you don't take it for granted, and for that, you need to master a second language.

As a trained (and formerly professional) linguist, I vehemently disagree.

(1) Linguistics, as you seem to allude to, is the study of language as a phenomenon, not the study of languages. It includes fields like the study of acoustics (the intersection of the physics of sound and the biology of the ear / vocal cords), and neurolinguistics (the neuroscience of language) that have nothing to do with sociolinguistics or second language acquisition. [0]

(2) The Sapir-Whorf hypothesis in its strong formulation is pseudoscience, and is rejected as such by the academic mainstream of linguistics. Journalists love it because it flatters them in their role as word-smiths, so there is sadly no end to the ink being spilled over it.

(3) Most crucially, any linguist worth their salt can work in multiple languages without necessarily speaking them. I can understand the sandhi phenomenon in Vedic Sanskrit without speaking Sanskrit, because that's sort of the job of a linguist? People can talk about 'sun and moon letters' in Arabic, and I can go - cool, assimilation of coronal consonants. I don't need to weep over Arabic poetry before realising this, this is just the basic tradecraft of a linguist. Same as I can understand lambda functions as a concept, without needing to know how they're implemented in <insert programming language here>.

In my experience, linguists enjoy other languages and often speak them, as I do, but it's rather ignorant to assert it's "professionally totally unacceptable" for, say, a neurolinguist to be monolingual. It suggests an exceedingly narrow view of the field.

[0] Here's a fun example of neurolinguistics. Consider the sentence 'I drinks a tall.' Approx 100-200 ms after reading this, an electrical signal (simplifying for clarity) fired off in your left frontal lobe, near the section of your brain called Broca's area. That signal, the Early Left Anterior Negativity (ELAN), is associated with syntactic violations (in this case, the use of 'tall' without an accompanying noun). Approx. 300-500 ms after reading this sentence, a second impulse, the Left Anterior Negativity (LAN) fired off in the same area of your brain. This impulse is associated with morphological violations (in this case, the use of 'I drinks' rather than the expected 'I drink'). If I said 'He drinks a tall', you'd just get the ELAN, but no LAN. If I said 'I drinks a tall glass of water.', you'd get the LAN but no ELAN. Fascinatingly, you haven't actually parsed the meaning of the sentence at the time either of these fired - that comes later, at around 600 ms (the P600 signal, in a different part of the brain). This kind of work is very much the domain of linguistics, and clearly not contingent on learning lots and lots of languages. (Enjoyable though that may be!)