Did the writers or producers comment on why this change was made?
No, they did not. So, we may never know.
What we know for sure is that many fans complained, both sides, starting from season 2: some thought it should have been the original language, translated, with subtitles. Some prefer the 100%-english version as it makes it less difficult to follow the action, as they don't have to read while missing some parts of the screen or details.
The writers made the task of watching and enjoying the series much better for its viewers by having the cast of various nations talk and converse mostly in English. It becomes a hassle for a few viewers of reading the subtitles and watching what is going along as well. Even if there are subtitles for a few scenes that showed us the past, they were kept minimal and had a decent time space in the dialogues. It's review time
I loved Season One, and was really looking forward to this second Season, but so much of the action is in Spanish, that I simply could not follow the plot. And yes, there are subtitles, but they went by so fast that I could not keep up with them, and I'm a fast reader. Plus, there are some scenes in Russian, which were frustrating for the same reason more - full review
By far the biggest hurdle to overcome in watching this season, and what has put me off entirely, is the complete lack of realism in any scene involving non-native-English-speaking characters when there are no English speakers (Americans or others who might realistically speak English just as a common second language) in the scenes. A swearing-in ceremony at the Kremlin where everyone is speaking English? A meeting in Prague, in the office of Czech President Alena Kovac, with only Czechs present...and they speak English?
Even if the following are opinions, they're pointing out the flaws (in whichever category you wish to sort them: by design, by cost, by convenience...). These are excerpt from discussions on quora, but they are more and similar topics in other media:
One possible reason is that it may have been a creative decision to make the show more accessible to a wider audience. By having all characters speak English, it could have made the show easier to follow for viewers who may not be familiar with the languages spoken in the previous seasons. This could have been particularly important as the show gained popularity and attracted new viewers.
Another reason could be related to the production logistics of filming with foreign languages. Shooting scenes with different languages can be complicated and time-consuming, particularly if multiple languages are involved. By having all the characters speak English, it may have streamlined the production process and allowed for a more efficient filming schedule.
It's also possible that the decision was influenced by feedback from viewers or producers, who may have preferred an all-English approach for various reasons, such as better ratings or increased commercial success.
One plausible reason as to why choose English as the movie language is the difficulty to cast characters that speak the specific foreign language used in the scene(s), or the difficulty to make it plausible. Switching to (bad) English with strong/fake accents can do the job, even if people keep complaining about that.
The decision to revert to English dialogue for characters who should be speaking other languages did not sit well with many fans of the show. While the show's producers have not given an answer as to why this decision was made, it may be because several of the major Russian characters are played by non-Russian actors. In some ways, this does mirror "The Hunt for Red October," where English was being spoken on a Russian submarine. Looper