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.

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