Can I create slug with unicode letters since all the major browsers support unicode urls (so it won't be transformed into the utf8 equivalent), or should I always use English letters?
For example: If I want to create a slug out of the words "Buenos días", should it be http://example.com/buenos-días or http://example.com/buenos-dias?
If you're mostly dealing with Latin characters
Only English (or actually Latin characters) is much better for SEO. I have noticed that some of my URLs that contain stuff like ä lead to Google getting confused because of URLs like /g%C3%A4stebucheintrag. In the long run this can really add up.
So if you have the choice, stick to plain English characters.
Of course there's also the typing issue, but not many people type addresses these days.
If you're mostly dealing with non-Latin fonts
E.g. Japanese, Hebrew, Chinese, Arabic, go for the URL that people may still be able to type. This will also be better for SEO than transliterated URLs, which is the opposite from the above case.
Sticking to just english letters is a better choice.
Not everyone has a keyboard that would allow them to quickly type in a letter with a symbol on top of it. Also, english letters are compatible with every web browser made on the planet including text-based browsers.
If however, you only wanted to limit the audience of your website to just a few individuals with compatible web browsers and special keyboards and/or people who have time to copy and paste special letters into a web browser, then you can use any kind of letter in a URL.
