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. You could also open the
Normal.dotx to change the default options when opening MS Word
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
' toggle_statusbar Macro
CommandBars("Status Bar").Visible = Not CommandBars("Status Bar").Visible
If only the language section is hidden, right click the status bar and enable it