Ideally, we should judge our professional interactions based on the merits of the arguments one puts forth.
However, a person’s comfort in speaking or writing in a particular language impacts the merits of the argument they put forth in that language. Even if you’ve studied the language, if you’re not fluent, sometimes you just can’t come up with the words to clearly convey your message in that language.
And this negatively influences your counterpart’s perception of you. It’s difficult to distinguish between someone who doesn’t know and someone who has difficulty communicating what they know.
As a result, being fluent in a language is a big asset to achieving professional success in a country where business is done in that language. The more communication a particular role requires, the greater the value of this asset.