Vietnamese language appears when resetting iPhone to factory settings

Vietnamese is a supported language. When you first set up an iPhone it lets you select language and region, right after the Hello screen. If you didn’t see this the phone had been set up previously.

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If you don’t want vietnamese on the preferred language list or the keyboard list or checked on the dictionary list, just remove it.

iphones are built to be usable in dozens of languages regardless of where they are sold. Sometimes settings change with no good explanation.

Are you having any operational problems with your phone?

Every language supported by Apple is available on every iPhone. So I could select Vietnamese here in the US, for example. I thought you had done a factory reset, but that doesn’t change the language selection dialog. As you are in a part of the world where Vietnamese is spoken it isn’t unreasonable that it would be on a list of preferred languages. I would guess that it would also have Chinese (Traditional and Simplified), Korean, Thai, and possibly Hindi on the list of preferred languages. I have Japanese here in the US, along with Korean, Hindi, Chinese, Thai, Vietnamese and many others.

Go to Settings/General/About and report what it says for Model Number.