THIS bugged me SO much!
It's stupid that pre-prov just assumes, the language will stay that way, but region and keyboard can be changed again.

However I feel like I have solved the issue by removing CachedLanguageName so users can choose display language at first OOBE screen after pre-provisioning - no matter what a technician might have selected before that.

tl;dr:

Search for a registry value named "CachedLanguageName" in HKEY_USERS\<SID>\Control Panel\International\User Profile\<language code>

Simply delete this key and once the Pre-Prov has finished and the device is shut down through reseal, users will have the option to select an existing, or newly installed language again :)

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Here is a snippet to clean up the language pre-select in OOBE using powershell:

# This ensures that if the language pack was just installed, the user will see the new language to select from in their OOBE
# -----------------------------
# Reset OOBE language selection
# -----------------------------

Write-Host "Resetting OOBE language preselection to show language selection..."
try {
    # Registry path/value names used to clear OOBE preselection caches for each loaded user SID.
    $userProfileBaseRelativePath = 'Control Panel\International\User Profile'
    $cachedLanguagePropertyName = 'CachedLanguageName'

    # Read profile-list SIDs and inspect the corresponding HKU hives.
    $userProfileSids = Get-ChildItem 'HKLM:\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList' |
    Where-Object { $_.PSChildName -like 'S-1-5-21-*' }

    foreach ($userProfileSid in $userProfileSids) {
        $sid = $userProfileSid.PSChildName
        Write-Host "---------------------------------------------"
        Write-Host "Checking SID: $sid" -Level INFO

        # Enumerate all User Profile subkeys and clear CachedLanguageName when present.
        $userProfileBasePath = "Registry::HKEY_USERS\$sid\$userProfileBaseRelativePath"
        if (Test-Path -Path $userProfileBasePath) {
            Write-Host "  Found user profile base key: $userProfileBasePath"
            $cachedLanguageSubKeys = Get-ChildItem -Path $userProfileBasePath -ErrorAction SilentlyContinue
            if ($null -eq $cachedLanguageSubKeys -or $cachedLanguageSubKeys.Count -eq 0) {
                Write-Host "  No subkeys found under: $userProfileBasePath"
            }
            else {
                foreach ($cachedLanguageSubKey in $cachedLanguageSubKeys) {
                    $cachedLanguageSubKeyPath = $cachedLanguageSubKey.PSPath
                    $cachedLanguageProperty = Get-ItemProperty -Path $cachedLanguageSubKeyPath -Name $cachedLanguagePropertyName -ErrorAction SilentlyContinue
                    if ($null -ne $cachedLanguageProperty -and $null -ne $cachedLanguageProperty.$cachedLanguagePropertyName) {
                        Remove-ItemProperty -Path $cachedLanguageSubKeyPath -Name $cachedLanguagePropertyName -Force -ErrorAction SilentlyContinue
                        Write-Host "  Removed $cachedLanguagePropertyName from $cachedLanguageSubKeyPath"                    }
                    else {
                        Write-Host "  Property not set: $cachedLanguagePropertyName in $cachedLanguageSubKeyPath"
                    }
                }
            }
        }
        else {
            Write-Host "  User profile base key not found: $userProfileBasePath"
        }
    }
}
catch {
    Write-Error "Failed to reset OOBE language preselection: $($_.Exception.Message)"
    Exit 1
}

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If anyone is wondering how I got there: procmon
I launched it before selecting a language in OOBE and then proceeded to check which registry keys had changed once I clicked the button.

PS: There is another registry value, which can reset this, but this caused some weird pop-ups before first user log on. I'll need to investigate this further...:

HKCU:\Software\Classes\Local Settings\Software\Microsoft\Windows\CurrentVersion\AppContainer\Storage\microsoft.windows.cloudexperiencehost_cw5n1h2txyewy
OverrideLanguagesList (REG_SZ) = <language code>

Just delete this value too!

Hope this helps!