For DNN 4.x

Step one:

"Admin->Site Settings -> Languages"

uncheck

• Enable Browser Language Detection For Current Portal?
• Disabling this option will show the site in the default language of the site.

Setp two:

"Admin->Site Settings"

in "Site Settings" "Advanced Settings->Other Settings->Default Language", switch to the language you want.

For DNN 5.x

From the Languages module, click the Edit Language link below the language you want to enable/disable to change the setting.

Then, you can disselect the “Enabled” option.