How does DuckDuckGo know my native language even though I am using a VPN in a country with a different language?

One possible explanation is that DuckDuckGo is using the headers that are sent in your request to determine their display. For example, it is very common to use the Accept-Language header to determine in which language a webpage should be displayed. This header is set by default in all modern browsers based on the language preference settings. My browser, as an example, sends Accept-Language: en-US for all requests, letting the target site know that they should attempt to send back US based English if possible. This does not require any sort of tracking to be used.

If you visit https://duckduckgo.com/settings you can see what the language settings are. The default language is Browser preferred language

I had the same problem: Google always responding in my native language, even though I used a VPN elsewhere in the world. I wiped all cookies and so on....to no avail... and then I found that I had an IPv6 leak! Google was using IPv6, and saw my home IP as usual. It wasn't "tracking" me, I just contacted Google straight around my VPN.

Do the test here: http://ipv6leak.com/

When I blocked ipv6, google responded in the language of the country of the VPN address.

Maybe you have a different problem, but have you checked this ?

I think @punpun1000 is right, you can easily test your browser language by visiting this website - https://browserleaks.com/ip or just simply run this JavaScript code:

alert(navigator.language);//works both in Mozilla and chrome
alert(navigator.language);//works both in Mozilla and chrome
alert(navigator.userLanguage);// Works in IE

More about regions and languages visit https://duck.co/help/settings/regions and https://duckduckgo.com/params