How to use :

1. Create a folder named ro_RO inside oc-content/plugins/invoice/languages  on your hosting.
2. Unzip ro_RO folder
3. upload the theme.mo / theme.po files.
4. change the language to Romanian in osclass settings.

@StefanGr
Thanks for naming convention update, just 2 comments there:
- word "plugin" is redundant (it should reflect plugin folder name )
- po & mo files must be packed inside language folder.... so you move it to ro_RO and then zip this folder 

Anyway thanks a lot, file was uploaded, you are helping Romania community a lot