On a Polish website where the title and content of the "Services" page is all in Polish, should I use an English permalink (
www.example.com/services) instead of a Polish permalink (
www.example.com/usługi) for better SEO?
I will keep the long story short.
Keep it in English If the visitors are searching your website by using English keywords.
Keep it in Non-English If the visitors are searching your website by using their native language as you mentioned.
SEO will work for both, but what you need to do is to judge the audience and decide whether you should go for English or Non English. And once decided stick to the plan. Do not mix English and Non English content together as it will confuse the search engine.
Permalinks, like all other site content, should be in the native language of the website if possible.
As far as I know, there is no SEO benefit to having permalinks in a different language from the website content, and it would seem to me to be a poor choice regarding user experience.
If you think about it, your Polish-speaking customers are not going to be searching for "ExampleCompany services", they are going to be searching for "ExampleCompany usługi" on a search engine that gives them Polish results, so the English URLs will look out of place in the SERPs.
As per my knowledge, Google confirms in their 2018 ranking algorithm updates that URLs are not a ranking factor but there is a catch that it will be best your permalinks are short as possible... this benefits in google ranking and also to protect from future updates... you should add your main keyword in permalinks.... No language issue will be faced there but advised to use permalinks in that language which traffic or people you’re targeting.