By my knowledge of English language - word “costume” have several meanings and most of them are not associated with meaning “costume” you are referring to (like Halloween costume or the way to make fun/parody of said set of cloths). Costume in English dictionary means also attire, cloths, outfit and I haven’t used it as offensive way to call national/traditional cloths of other cultures.
More explanations of word “costume” can be found in Oxford Dictionaries ( http://www.oxforddictionaries.com/definition/english/costume ) or Cambridge Dictionaries Online ( http://dictionary.cambridge.org/dictionary/british/costume )
Furthermore the common used by everyone sites and internet tools such as Wikipedia and Google also use the word costume as common word for cloths ( http://en.wikipedia.org/wiki/Folk_costume). Even my own country’s traditional outfit in English is called “National costumes of Poland" http://en.wikipedia.org/wiki/National_costumes_of_Poland ) and this name is not connected with “funny parody” of our traditional cloths.
I’m sorry if you though I’m using term “costume” as meaning of “Halloween costume”, but I’m not. Would I used term “costume” if it’d be rude or offensive and if I’d know that it’s also used to describe my own traditional costume? Ofc no. Would I use it as rude way to call other people traditional cloths? EVEN MORE NO.
I’m sorry that you got confused - indeed if you search Google for terms “kimono costume” you will find mostly Halloween costumes, but if you search for “Japanese/Korean/Vietnamese ect. costume” you will find mostly normal traditional cloths of said countries.
Nevertheless from grammar/linguistic point of view, term “costume” is correct and for sure is not used by me as offensive, sarcastic or rude toward other culture.
Hope this explanation will remove your doubts.
This blog is devoted to Asian national costumes (East, South, Southeast Asia and Pacific). Enjoy.