Hello,
Many thanks for your contact. The Serbian has two language code: Serbian Latin sr-YU and Serbian Cyrillic sr-RS. You can find the language folder of the plugin here: plugins\system\onestepcheckout\language

For example, if you want to create language file according to the code of Serbian Cyrillic, you will need to create as follow:
plugins\system\onestepcheckout\language\sr-RS\sr-RS.plg_system_onestepcheckout.ini

I hope it will be useful,

Best regards,
Ken

Thank you. In meanwhile, I found that path and finished translation.
Best regards, Ljiljana

Hi,
Many thanks for your reply. If you have any questions, feel free to submit a ticket to us, the team will better support: http://cmsmart.net/your-profile/user_list_ticket
Best regards,
Ken