I have a multlanguage site, and I need to add a link of a category in phtml file. How can I do it?
This won't be optimal and elegant solutions since you provided no information about layout structure so generally you can generate any link to a category on frontend like this:
$categoryUrl = Mage::getModel('catalog/category')->load($categoryId)->getUrl();
It is very likely however that this url is already in the menu. I would create an observer for
page_block_html_topmenu_gethtml_after and fetch the category data from the Menu object passed to that event.