Alternate language not working?

[ca_rem]

Everyone, I have 3 questions about Multiple Language User Interface (MUI).
1/ When I choose my default language “Vietnamese”, and I set the Alternate Language(s) is “English”, the Language switch from the default to the alternate. But when I reverse. choose “English” is default, and Vietnamese is alternate, the Language skip default. Why?
2/ Will using MUI affect Sharepoint configuration via SPD 2013 and Sharepoint Powershell Management? Or is there any significant effect on it?
3 / If the admin only wants to use English, but for the end user to use Vietnamese language, how should I do it? Select the default language is English or Vietnamese? And the alternate language opposite? Or is there another way more effective?

Language SettingNothing Change

[#2]

You are hitting here one of the pain points when working with SharePoint and trying to use the multilanguage features because SPO not only takes into account the default language of the user but also the language of the OS and the browser so sometimes it's a little bit tricky to get it working

[#3]

@Juan Carlos González Martín I figure out that when I change my Language of Office 365 (In Delve -> Update Profile), you can see more detail here: https://support.office.com/en-us/article/change-your-personal-language-and-region-settings-caa1fccc-....

The Language of Sharepoint switches correctly to my most priority language.