You can't have spaces in URLs. K2 converts spaces to the appropriate characters according to the URL specification  www.ietf.org/rfc/rfc1738.txt . Then it's up to the browser you are using to convert them back and display them correctly. Most modern browsers do that.

1. Chrome : URL shown on item page (when hovered) : "mysite.com/photos/itemlist/tag/Test Tag" but will load: "mysite.com/photos/itemlist/tag/Test%20Tag"
2. Edge : URL shown on item page (when hovered) and will load : "mysite.com/photos/itemlist/tag/Test%20Tag"
3. FireFox : URL shown on item page (when hovered) and will load : "mysite.com/photos/itemlist/tag/Test Tag"

# remove spaces from start or after / RewriteRule ^(.*/|)[\s%20]+(.+)$ $1$2 [L] # remove spaces from end or before / RewriteRule ^(.+?)[\s%20]+(/.*|)$ $1$2 [L] # replace spaces by - in between RewriteRule ^([^\s%20]*)(?:\s|%20)+(.*)$ $1-$2 [L,R]

"In Joomla!, each component is responsible for handling its own SEF URLs. Therefore, as the developer of a component, you will have to create your own router to allow your component to use SEF URLs."

"On the other hand, if you make distinct menu items, then you can create your own URL alias, in which case you can of course insert a dash between a tag's words."

You need to add a language pack in Joomla for sure with a language that handles these special characters.

Then and only then can Joomla (and K2) perform any such character replacements.