Duplicate the SetLanguage_EN_US function and change the name to your ISOCode
Example: English (United Kingdom) | en-gb
We duplicated the SetLanguageEN_US function and changed the name to SetLanguage_EN_GB
Recently created SetLanguage function in Localization.cs
Now translate every string to your new language
In the SetLanguage function, add another else if using your language ISOCode.
SetLanguage function in Localization.cs
In Views folder locate SettingsWindow.xaml.cssearch for GetSelectedLanguageIndex and add your language ISOCods
Duplicate the code from SetLanguage_en_US_MouseDown function
And place a correct name to the function, and also change the string of the language.
Placing new correct values
In SettingsWindow.xaml search for comboBoxUILanguage element and search for the items and add a new one for your language.
Remember to change the PreviewMouseDown refering to the above function.
That's all!
Now run your Launcher and change the language to the new one!