Handbrake Nightly its a free soft ware.
Direction:
1. Open HandBrake.
2. Open Source File (MKV)
3. Click Subtitle tab.
4. Click Add.
5. Choose Burn In.
6. Click and choose a Destination Folder.
7. Start.
That's it. It changes MKV to MP4 by default. Unless you wish to change the size or file size is another thing.
Credit goes to
Jox who introduced it to me.
Here is our discussion.