Go to Tools-->Options-->Languages-->[Navigate to language]-->Formatting-->hit the "Edit..." button.
Go to Indent-->General and change the "Indent with" dropdown to "Tabs" and click "OK" and "OK" to exit the dialogs.
Note: "Plaint Text" mode is used for files that are not in the list or have no extension.
Tools-->Options-->Languages-->Miscellaneous Languages-->Plain Text-->Formatting-->check "Indent with tabs".
