DGIndexNV can't extract subtitles, there are two common ways:
1. MakeMKV > mkvextract, StaxRip and RipBot264 fully automate mkvextract. DGDecNV, ffms2, DSS/DSS2 all can open MKV containing AVC/VC-1/MPEG-2. Works equally and great for both DVD and Blu-ray.
2. eac3to demuxing, this is a command line tool, most GUIs make it easy to use, StaxRip, RipBot264, MeGUI, HD-DVD/Blu-Ray Stream Extractor and many others.