YouTube allows you to upload custom subtitle files when playing a rented movie via the “CC” settings. If you rent Avatar on YouTube, click the gear icon > Subtitles/CC > Add subtitles > Upload your .srt Bangla file.

If your subtitles are off by a few seconds, you don’t need to download a new file. Use VLC Media Player:

Sometimes, Bangla subtitles are created for a different video release (e.g., 3D version vs. Theatrical version). If the subtitles are out of sync: