When I came, most Japanese people thought that it was practically impossible for any non-Japanese to conquer the language. People were very impressed if a foreigner mangaged to say a few words of greeting in the language, but few expected one to be able to do more than this. The thought that foreigners could really understand Japanese, in the same way that they understood their own languages, seemed hard to comprehend to most Japanese. This attitude itself, of course, tended to be self-confirming. As nobody thought the average foreigner could speak Japanese, nobody spoke to foreigners in Japanese.
I have two questions. #1 Why , , is used? What effect do the commas have?
#2 What does self-confirming mean?
Which commas do you have questions about? If you have a part of the sentence that is added for additional information and it is not necessary for the grammar of the sentence to work, you set it off with commas. This is also known as a parenthetical remark.
Self-confirming - their own actions ensured that what they said come true. "Self-fulfilling" is a more common expression.
The commas I mentioned are "," the same way that they understood their own languages"," What effect do these have when they are put there or not?
Those commas are probably optional. I believe the author used them to set off "the same way that they understood their own languages" as a parenthetical explanation of what he or she meant by "really understand."
self-confirming is used as an adjective. It describes their belief that no one could really learn Japanese.