Jennie, if you decide to pursue a career in academia, you will encounter quite a lot of such people, due to the largely international nature of the profession. Well depends where, but in top places in the US definitely. Many great studies and lectures, but the language limitations or otherwise poor speaking skills of the presenter makes it quite hard to follow, or may kill it even if you wish.
I actually experienced this myself when I fist started grad school many years ago, just fresh out of college in my home country where I never had great opportunities to learn well and especially practice the English language. Yet I would challenge myself and went to grad school in a country where I had absolutely no knowledge of the local language (not English), and while my written English was not bad, I could hardly speak it as our working language. And I had to start giving regular presentations from the beginning. I really struggled with it for a while. I would usually put a lot of text on my slides so at least the audience could read my message. My supervisor actually told me that my strategy was very good. I picked it up pretty fast then, maybe after half a year of suffering with the worst.
So I always feel for people like the professor you mentioned... some people just don't have very good abilities to learn foreign languages and will never develop a proficiency even after years or decades. But can still be brilliant in what they do; just very hard for them to "sell" it due to these limitations. Same for writing. I know scientists who keep using a professional editor for their papers even after decades because they just can't develop the necessary English skills.