This script uses the mkvsrtadd to process a batch of mkv files at once.

mkvsrtaddbatch.sh

#!/bin/bash

# This script needs "mkvsrtadd" script
# (available on https://gist.github.com/samirfor/6dd0d0e8e2344ba3d093)
# to work properly. Make sure you have it in the same path of this script.

self=$(basename "$0")
selfdir=$(dirname "$0")
PWD=$(pwd -P) # resolving symbolic links
usage="[i] Usage: $self \"pattern\" \nBe sure pattern is into double quotes."

# Checking parameter
if [ -z "${1+x}" ]; then
    echo ""; echo "$usage"; echo ""; exit 1
fi

SAMEFOLDEROUTPUT=0
if [[ ! -z "${2+x}" && "$2" =~ [Ss] ]]; then
    echo "Same folder output files [put all output files in current folder]<ON>"
    SAMEFOLDEROUTPUT=1
fi

# Displaying the files match by pattern
# ignoring files already processed (*.br.mkv)
echo
echo "INFO: The following files will be processed ..."
echo
find . -type f -name "$1" ! -name "*.br.mkv" -print
echo
read -p "Are you sure? [Y/n] " -n 1 -r
echo    # (optional) move to a new line
if [[ $REPLY =~ ^[Nn]$ ]]; then
	echo "INFO: Aborted."
    exit 1
fi

# Do it by a pattern
find . -type f -name "$1" ! -name "*.br.mkv" -print0 | while IFS= read -d $'\0' file; do
	echo
	echo "INFO: Processing '$file' ..."
  if [ $SAMEFOLDEROUTPUT == 1 ]; then
    bash "$selfdir/mkvsrtadd" "$file"
  else
    DIR=$(dirname "$file")
    FILE=$(basename "$file")
    cd "$DIR"
    bash "$selfdir/mkvsrtadd" "$FILE"
    cd - 1> /dev/null
  fi
done
cd "$PWD"