I assume you want to change the languages' URL code (en, bg, ro).
Then you need to go:
Extensions -> Languages -> Content Languages
Select/Click the language you want to edit and then change the field called: URL Language Code.
ATTENTION: Changing the URL language code, will make all your old links to result to 404 errors.
So, if you change the ro to romania, then the previous url:
example.com/ro/page will be accessible only at
example.com/romania/page and the old url will be a 404.
As a further note - if you want your default/main language to be displayed without a language url code (e.g. let's say english is your default lang and you want your english pages have no
/en in their URLs, then you need to edit this on the System - Language Filter plugin.
Go to Extensions->Plugins, do a search filter "Lang" - you will find 2 plugins. Select/Edit the Language Filter plugin, and set the field Remove URL Language Code to Enabled (YES).
This setting will remove the language URL code and redirect all the pages of your default language.