FIeld value only in SEF URL

How to show only the field value in url?

FOr example is the name is city and the fiedl value Paris iWant the url to be www.xxx.FR/tag/paris and not www.xxx.FR/tag/city-paris

Which file do i need to modify?

Thanks

That's not a quick modification to do. There's no single file I can point you to.
Consider that Custom Properties NEED to know the name of the field to be able to distinguish which tag is which. See the following example:

Socks => (Red, Green, Blue)
Shirt => (Red, Green, Blue)

(or, in you example, you would not be able to tell Texas => Paris from France => Paris

However, if you want to try the modification, here's where you have to start:
- helper.php: showTags function writes the urls
- helper.php: getTagByName function, converts a text tag into it's numeric equivalent
- router.php: SEF urls encoding / decoding