Browser language properties have been used as a device fingerprint element since fingerprinting has existed. While it's relatively generic there are a few interesting features that make it useful. The defacto standard has been to use the language property to return the browser language, and up until recently Microsoft's Internet Explorer did not use this. Instead, IE gave you options for system, user and browser settings. Current versions of Edge now support the language property, but will also return the legacy properties. This will allow you to distinguish browsers even if the user-agent has been modified.
How It Works
In order to get the best print possible from languages we'll have to make several calls in order to capture as much entropy as possible. It is also important to pay attention to the case of the returned data as that can give you additional entropy. Below we'll define the properties available.
The language property returns the language version of the browser. It should be noted that this is not impacted by the setting within the browser, but by the installation selection. This is supported by all major browsers with the exception if IE 10 and lower.
The languages property returns a string of supported languages, while the above language property only provides the first in the list. This is supported by all major browsers with the exception of Safari 8 and lower, and Microsoft Explorer.
There are three additional properties that are proprietary to Microsoft, and are returned in all versions of their browsers. They are userlanguage, browserlanguage and systemlanguage. The userlanguage property reflects the setting in the Regional Options under control panel. The browserlanguage property refects the language of the operating system reguardless of the installed version of Windows Internet Explorer. The systemlanguage property reflects the language edition of the installed operating system.
Entropy Estimate: 10.1 bits
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NavigatorLanguage.language (Jun 30, 2017). In MDN web docs. Retrieved September 01, 2017, from https://developer.mozilla.org/en-US/docs/Web/API/NavigatorLanguage/language
NavigatorLanguage.languages (May 11, 2017). In MDN web docs. Retrieved September 01, 2017, from https://developer.mozilla.org/en-US/docs/Web/API/NavigatorLanguage/languages
browserLanguage property (n.d.). In Microsoft Developer Network. Retrieved September 01, 2017, from https://msdn.microsoft.com/en-us/library/ms533542(v=vs.85).aspx
systemLanguage property (n.d.). In Microsoft Developer Network. Retrieved September 01, 2017, from https://msdn.microsoft.com/en-us/library/ms534653(v=vs.85).aspx
userLanguage property (n.d.). In Microsoft Developer Network. Retrieved September 01, 2017, from https://msdn.microsoft.com/en-us/library/ms534713(v=vs.85).aspx