People all over the industrialized world learn English, no matter how different their own native languages might be from English. So why is it acceptable for English-speakers to go to other countries, settle there, and not learn the local languages?
This is a common problem among most expats, especially English-speaking expats, but it's a problem that's common among expat rabbis also.
One rabbi I spoke to said of kashrut in his country "It's all bishul akum anyway! Just shop at Costco!" (It's not all bishul akum [food that in order to be kosher must be cooked by a Jew]! Plenty of things like vegetables, cold-pressed oils, soy products, are not subject to the laws of bishul akum. Wine and cheese and bread are not native to that rabbi's country.) Another person said that rice is something that can't be trusted because it's not checked for bugs. If a bag of rice has a bug in it, then you know! There are strings of sticky web all through the bag, and you have to throw it out! Rice isn't like lettuce or broccoli that can be washed and checked! But unless you're used to a rice-based diet, you don't know that. Costco!? Checking rice!?! Copepods in the water! Give me a break!!
So when observant Jews go to other countries, they often need to be in contact or in dialogue with the local culture, which at least means knowing what foods can or cannot be eaten; which foods have certain kinds of significance; what the socially acceptable way to eat certain foods might be.
This has social and ethical implications also. People approach rabbis with ethical dilemmas. In order for the rabbi to effectively deal with that person's question, the rabbi must have a working knowledge of social conventions and expectations of what is considered proper and ethical behavior in that host country, and allow the halachah to take that into account. Too often, American or Israeli rabbis establish themselves in other countries without knowing the language, and pasken based on their previous knowledge, and on second or third-hand hearsay about foods and social norms of their host countries.
Being out of touch with the realities of his host country make him not only unqualified to give reliable psak halachah, but also make him less relevant to most observant Jews. People are forced to pasken for themselves because they know that the local rabbi doesn't know what's happening around him. Another possibility, equally dangerous from a religious point of view, is that observant Jews who are keeping kosher based on their own real knowledge of the local foods, that the rabbi does not share, are ostracized because their kitchens are declared unkosher, and so the more machmir Jews in the community who follow the local ignorant rabbi won't eat at other kosher Jews' homes.
Any rabbi taking a post abroad without learning that country's language to fluency, without learning the customs and values of that country's culture, is setting himself up for failure, for irrelevance, and puts himself in a position to mislead his community.