It appears that translation to the Serbian Latin Alphabet is not officially supported by the Cloud Translate API, as discussed in this recent issue. Therefore it’s not assured that any possible workaround will be functional or reliable. You can see the list of supported language codes for translation here.
You can, however, submit a Feature Request to the public Google issue tracker for Cloud Translation API. The higher the number of users who bring attention to this request, the more likely it is for it to be eventually built into the API.