Quality of the subtitles depend on the company the production company contracted out the subtitles to be created.

The company that puts together the subtitle file does so using the provided script with time stamps provided.

If multiple languages are needed the production company hires companies to do the translation and other companies might be hired to put together those subtile files for those specific languages they specialize in.

Some companies in some regions may have multiple departments that specialize in two or more of these feilds.

Delux entertainment for example does audio mixing and also has a subtitle department.