It works in all OC 3.x.x.x versions, from 3.0.0.0 to 3.0.3.7

Easy installation through OpenCart extensions installer.

Go to Extensions >> Installer and press the Upload button.
Find the zip file in your computer and upload it to your OpenCart.

Go to System >> Localisation >> Languages and press the Add New button.
Enter the following values in the text fields, or choose from the select menus:

• Language Name: Română
  
• Code: ro-ro
  
• Locale: ro_RO.UTF-8,ro_RO,ro-ro,romanian
  
• Status: Enabled / Disabled
  
• Sort Order: 0 (if you want Romanian language to be listed as the first language)
  

Press Save button.

Demo: https://opencart.extensions.one/language/romanian/