Here's how I handle subtitles:
Follow Mindbomb's stickied guide at the top of HTPC forum: Advanced MPC-HC Setup Guide. After you've done that, open up MPC-HC. Now go to View, Options, and then select External Filters, from the left side. Double click on LAV Splitter. Under Audio and Subtitles add all of the languages that appear in every movie you own. Mine has the following in both Audio and Subtitles: eng,jpn,spa,spn,fre,ger
Change Subtitle Selection Mode to "Only Forced Subtitles." Put a check mark in the box for "Only with Matching Languages." Hit OK.
Now Click on the section "Keys" on the left side of MPC-HC. Scroll down and click on "Next Audio." Move the menu bar to the right. Click on "App Commands." Select "Media_Channel_Up," or whatever button on your remote control you want to map it to. Do the same for "Next Subtitle," but set the App Command to Media_Channel_Down.
After you rip a movie, using MakeMKV, play the MKV, to determine which subtitles are forced subtitles and which are regular English subtitles. Once you've figured that out, drag and drop the MKV into the program, MKVMerge.
Let's say you have an English language movie with a few lines of dialogue in French. Make sure that the English audio track is marked as English. Set the normal subtitles as English, and make sure that "Forced Flag Track" is set to No. Set the forced subtitles as English, also. But this time, set the "Forced Flag Track" to Yes.
Or let's say you have a Japanese movie with both English and Japanese audio tracks; and it has forced subtitles and normal English subtitles. Make sure the English audio track is marked as English, and that the Japanese audio track is set to Japanese. This next part will seem counter intuitive, but trust me on this. Set the normal English subtitles to Japanese, for the language, and set the "Forced Flag Track" to Yes. Set the forced English subtitles to English, for the language, and also set the "Forced Flag Track" to Yes.
Now whenever you watch a movie, using MPC-HC, it will default to the forced subtitle track. If I want to switch to normal subtitles, I just hit the Channel down button, on my remote. Hitting the same button again, will turn off subtitles. And hitting it a 3rd time, will return it to the forced subtitle track. I can also switch audio tracks using the channel up button.
It sounds like a lot of work, but it's not. It's just a few extra steps. Hope this helps.