First, context. The 2009 release of Avatar was a landmark. Unlike many films shot on 35mm film, Avatar was a digital 2K production (mastered at 1080p equivalent for stereoscopic rendering). While a 4K remaster exists for later re-releases, the 2009 Blu-ray represents the film exactly as audiences experienced it during its historic IMAX 3D run—before subsequent color grading changes or HDR re-grades.
If missing, external .srt files work fine. Avatar.2009.BluRay.REMUX.1080p.AVC.DTS-HD.MA5.1