The phrase "how do you say" is grammatically correct and commonly used by native English speakers to ask how to express a word or phrase in another language or to ask for the name of something. For example, "How do you say hello in Japanese?" or "How do you say standing mixer in Arabic?" are typical uses of this phrase. It is an accepted and understood way to inquire about translations or expressions.
Additionally, the pronunciation of "how do you say" sounds like "HOW DOH YOO SAY" with the phonetic transcription /haʊ dəʊ juː seɪ/ in British English or /háw də́w jʉ́w sɛ́j/ in modern IPA.
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