The ‘Forced’ subtitles are in the subtitle tracks on the disc in your language with the ‘fewest’ elements.
Those tracks MUST be ripped out in MakeMKV, flagged ‘Forced’ with the correct Language along with the audio track.
Here’s what the end result will be for embedded tracks:
General
Unique ID : 15125292195752520219533589625215761978 (0xB61067B17A6194128CFC3FCF25DBE3A)
Complete name : G:\TV - Sci-Fi\Star Trek Picard\Season 01\Star Trek Picard - S01E01 - Remembrance.mkv
Format : Matroska
Format version : Version 4
File size : 962 MiB
Duration : 44 min 18 s
Overall bit rate : 3 036 kb/s
Encoded date : UTC 2020-01-31 16:16:31
Writing application : Lavf58.26.100
Writing library : Lavf58.26.100
ErrorDetectionType : Per level 1
Video
ID : 1
Format : AVC
Format/Info : Advanced Video Codec
Format profile : High@L4.1
Format settings : CABAC / 4 Ref Frames
Format settings, CABAC : Yes
Format settings, Reference frames : 4 frames
Codec ID : V_MPEG4/ISO/AVC
Duration : 44 min 18 s
Bit rate : 2 650 kb/s
Width : 1 920 pixels
Height : 800 pixels
Display aspect ratio : 2.40:1
Frame rate mode : Variable
Color space : YUV
Chroma subsampling : 4:2:0
Bit depth : 8 bits
Scan type : Progressive
Stream size : 821 MiB (85%)
Writing library : x264 core 157 r2935 545de2f
Encoding settings : cabac=1 / ref=3 / deblock=1:0:0 / analyse=0x3:0x113 / me=hex / subme=7 / psy=1 / psy_rd=1.00:0.00 / mixed_ref=1 / me_range=16 / chroma_me=1 / trellis=1 / 8x8dct=1 / cqm=0 / deadzone=21,11 / fast_pskip=1 / chroma_qp_offset=-2 / threads=12 / lookahead_threads=2 / sliced_threads=0 / nr=0 / decimate=1 / interlaced=0 / bluray_compat=0 / constrained_intra=0 / bframes=3 / b_pyramid=2 / b_adapt=1 / b_bias=0 / direct=1 / weightb=1 / open_gop=0 / weightp=2 / keyint=240 / keyint_min=24 / scenecut=40 / intra_refresh=0 / rc_lookahead=40 / rc=2pass / mbtree=1 / bitrate=2650 / ratetol=1.0 / qcomp=0.60 / qpmin=0 / qpmax=69 / qpstep=4 / cplxblur=20.0 / qblur=0.5 / vbv_maxrate=62500 / vbv_bufsize=78125 / nal_hrd=none / filler=0 / ip_ratio=1.40 / aq=1:1.00
Language : English
Default : Yes
Forced : No
Color range : Limited
Color primaries : BT.709
Transfer characteristics : BT.709
Matrix coefficients : BT.709
Audio
ID : 2
Format : AC-3
Format/Info : Audio Coding 3
Commercial name : Dolby Digital
Codec ID : A_AC3
Duration : 44 min 18 s
Bit rate mode : Constant
Bit rate : 384 kb/s
Channel(s) : 6 channels
Channel layout : L R C LFE Ls Rs
Sampling rate : 48.0 kHz
Frame rate : 31.250 FPS (1536 SPF)
Bit depth : 16 bits
Compression mode : Lossy
Stream size : 122 MiB (13%)
Title : English
Writing library : Lavc58.46.100 ac3_fixed
Language : English
Service kind : Complete Main
Default : Yes
Forced : No
Text #1
ID : 3
Format : UTF-8
Codec ID : S_TEXT/UTF8
Codec ID/Info : UTF-8 Plain Text
Duration : 9 min 25 s
Language : English
Default : No
Forced : Yes
Text #2
ID : 4
Format : UTF-8
Codec ID : S_TEXT/UTF8
Codec ID/Info : UTF-8 Plain Text
Duration : 42 min 18 s
Language : English
Default : No
Forced : No
Menu
00:00:00.000 : :Amazon Original
00:00:06.006 : :Keep the game from ending
00:04:01.116 : :Activating a new weapon
00:06:55.081 : :Opening Credits
00:08:33.013 : :An important interview
00:16:38.164 : :Dahj finds Picard
00:22:06.033 : :A clue hidden in a painting
00:26:33.092 : :Focus and find help
00:28:08.020 : :The truth about Dahj
00:33:49.069 : :A visit to Okinawa Research Facility
00:40:38.061 : :Another sad story
00:43:17.095 : :End Credits
Note: in this state you can’t see the ‘elements’, but you can see there’s a LOT more time in the Full Track than the Forced track, eh?
Here’s how those same tracks would be ‘flagged/tagged’ as sidecar files:
A TV Show Library/
.....Star Trek Picard/
.......Season 01/
..........Star Trek Picard - S01E01.xxx
..........Star Trek Picard - S01E01.eng.forced.srt <---forced
..........Star Trek Picard - S01E01.eng.srt <---full
If the audio tracks are flagged with the correct language - in this case English - Plex will know what to do with them - otherwise Plex will guess and might even get it right - or not.
If your media already has all the tracks - including the forced track - you can 'flag and ‘tag’ correctly with MKVToolNix - and ‘mux-out’ unneeded tracks at the same time.
If your media contains no ‘forced track’ - you can try to get one here:
https://subscene.com/
if that plan falls through - you can re-rip and this time get the track you need.
Here’s a view from MKVToolNix for a file still hot from the ‘presses’ - that would be a complete mystery if all the flags and tags weren’t made…:
^^That’s also MKVToolNix’s ‘Header Editor’ - no remuxing required.
Instant gratification.
That won’t work if you’re ‘removing’ tracks - that’ll require a re-mux.
Last Note:
I always include a full track - UTF-8/SRT/Text based.
If there’s a Forced Track that always goes on first - the full track second - so if something horrible happens (20 times a day) and I forget to flag it, I know the Forced Track is the first one (until I can get it through MKVToolNix to tag and flag correctly).