The easiest way is setting the language by pressing Ctrl+A (or only the words in that language) then click the language button in the status bar > Mark selected text as and select the correct language

This way you can also check spelling in mixed-language documents but still avoid false recognizing spelling/grammar errors. Office 2013 and above has support for Vietnamese spell check.

If you want to select a default language for new documents, click it and select Set As Default. You can also change the spelling/grammar checking for any languages.

You can create multiple templates for English and Vietnamese to make it easier when editing a file in a language by creating new documents, Ctrl+A, change language as above and save the file as word template (*.dotx). After that you just need to open the template and don't need to worry about changing language again. Alternatively just open the Normal.dotx to change the default options when opening MS Word. You can also create separate styles for different languages and switch the style when switching language

In fact the best way is to use different keyboard layouts for different languages. Word has the ability to automatically change the typing language depending on the language of the current layout. Unfortunately the Vietnamese layout is so bad that no one uses it

Edit

All MS Word versions have a status bar at the bottom. If your status bar has been accidentally hidden you can toggle its visibility by the following macro

Sub toggle_statusbar()
' toggle_statusbar Macro

    CommandBars("Status Bar").Visible = Not CommandBars("Status Bar").Visible
End Sub

If only the language section is hidden, right click the status bar and enable it