I would prefer not to use the word "usually" in the sentence. We use "usually" to describe a habitual act, and the sentence's subject has the option to perform such an act at a definite frequency. It also implies a contrast to the alternative. For example, I usually go to the one on 14th Street for my daily morning coffee. And I usually take my coffee in the early morning. The former shows a contrast in location, whereas the time in the latter. Since knowing the culture of the target is merely an attribute or characteristic of a person, a better word choice could be "generally." It's okay with "often," but the expression "We often find someone who/(knowledgeable/attractive...)..." is more common.

Apologises I had some problem with the predictive text of the computer. See if you can find these places in the text.

It is understandable that you would be confused by this. Understanding well the meaning of "often" or "usually" will not help you choose between the two. The only thing that would help you choose would be the ability to read the author's mind and know what she is trying to say. It's not a fair question. Your choice of "often" is reasonable.

In much daily life talking and speaking you will hear natives interchanging "usually" and "often". in daily informal chatting, it makes no differences to both parties or all parties understanding. But for English language Exams that test your grammar knowledge and fully fluent and accurate English. You will be expected to know that "Usually" refers to an habitually habit. (which is or can be often). Often refers to something that happens often (in the context of the text) . Therefore language students "OFTEN" do not know the culture of the target language. It is inaccurate to say or suggest they HABITUALLY "do not know the culture of the target language". This makes more sense if you imagine a language student taking their first ever lesson. You can't say that this person is in the HABIT of "not knowing the culture of the target language". It will be the case that OFTEN these students will not know the culture of the target language. Therefor for an academic English language test the correct answer is "OFTEN"

Both are ok. If you had a list of words to choose from then maybe one of them fits better in another gap.