I would also be surprised if they question my English, as i'm a native speaker but my question referred to my partner who is Mexican and will be included on my application.
Right - well, I was a bit puzzled that you should be concerned about it, giving what you were posting!
However, there is even less reason to think that there will be a problem over the evidence of English offered since it's about your partner, the secondary applicant. When you apply under SMC, his work is completely irrelevant to the application, so INZ won't be bothered about his workplace and job description, and they're unlikely to meet or talk to him. Under SMC, everything depends on the main applicant's qualifications and job, and then proving the secondary's/secondaries' connections to him/her.
Re: English of native speaker, I've just remembered, my son's partnership Residence application was initially refused on the grounds that his English wasn't good enough.
He is a well-educated Englishman, who talks like a BBC newsreader, and the letter from INZ was in such bad, obviously 2nd-language, English, that it was barely comprehensible. He went straight to the INZ office, showed the letter, and explained the problems he had with it (rather eloquently, knowing him!), and the contrast was enough that the situation was all sorted out and the visa granted within a couple of hours. (That was 14 years ago, and they didn't seem to have the long queues there can be for 2pc back then.)