Windows Powershell functions to convert video files with mkvmerge from mp4 and avi to mkv and set all mkv default audio and video language to your desired language with mkvpropedit all in batch to be used for your PLEX server or something else.

Microsoft.PowerShell_profile.ps1

# Credit: https://www.github.com/jwallet

function mkv-set-language([string]$lang="eng") {
	Get-ChildItem -Path "." -Name -Recurse -File -Include *.mkv | ForEach { echo ">>> Getting file: $_"; Set-ItemProperty $_ -name IsReadOnly -value $false | & 'C:\Program Files (x86)\MKVToolNix\mkvpropedit.exe' --tags all:'' --delete title --edit track:v1 --set language=$lang --edit track:a1 --set language=$lang "$_" }
}

function to-mkv([string]$lang="eng") {
	Get-ChildItem -Path "." -Name -Recurse -File -Include *.avi,*.mp4 | ForEach { echo ">>> Getting file: $_"; $_ -match '(?<filename>.*)[.]'; $output = $matches["filename"] + ".mkv"; Set-ItemProperty $_ -name IsReadOnly -value $false | & 'C:\Program Files (x86)\MKVToolNix\mkvmerge.exe' -o $output --default-language $lang "$_" }; mkv-set-language $lang;
}

Here is the script if you want to do the same thing but leverage the Handbrake CLI. Open Powershell as Administrator then run the script with PowerShell.exe -ExecutionPolicy Bypass -File C:\Users\Username\Documents\WindowsPowerShell\Microsoft.PowerShell_HandBrake_profile.ps1

`$sourcePath = "C:\Video-File-Directory"
$handbrakePath = "C:\Program Files\Handbrake\tools\HandBrakeCLI.exe"

$videofiles = Get-ChildItem $sourcePath -Filter *.mp4 -Recurse
$filecount = $videofiles.Count
$i = 0

ForEach ($file in $videofiles) {
$i++
$origfile = $file.FullName
$newfile = $file.DirectoryName + "" + $file.BaseName + ".mkv"

$progress = ($i / $filecount) * 100
$progress = [Math]::Round($progress, 2)

Clear-Host
Write-Host "-------------------------------------------------------------------------------"
Write-Host "Handbrake Batch Encoding"
Write-Host "Processing - $origfile"
Write-Host "File $i of $filecount - $progress%"
Write-Host "-------------------------------------------------------------------------------"

$args = "-i `"$origfile`" -t 1 -o `"$newfile`" -f mkv -O --decomb -e x264 -q 22 --pfr -a 1 -E av_aac -B 160 -R Auto --x264-preset=fast --x264-profile=main --h264-level=`"4.0`" --verbose=0"
Start-Process $handbrakePath -ArgumentList $args -Wait -NoNewWindow

}`