Remove a language from an SXA site. Based on the "Add Site Language" set of scripts that comes with SXA.

Remove Site Language.ps1

# Functions Site Language/Remove Site Language

Import-Function Validate-PowerShell
    
Test-PowerShell

Try
{
    $ctx = Get-Item .

    Import-Function Remove-SiteLanguage
    Import-Function Show-RemoveSiteLanguageDialog

    $result = Show-RemoveSiteLanguageDialog $ctx

    Remove-SiteLanguage $ctx $result.Language | Out-Null
}
Catch
{
    $ErrorRecord=$Error[0]
    Write-Log -Log Error $ErrorRecord
    Show-Alert "Something went wrong. See SPE logs for more details."
    Close-Window
}

Remove-SiteLanguage.ps1

# Functions Site Language/Remove-SiteLanguage

function Remove-SiteLanguage {
    [CmdletBinding()]
    param(
        [Parameter(Mandatory = $true, Position = 0)]
        [Item]$Site,

        [Parameter(Mandatory = $true, Position = 1)]
        [string]$Language
    )

    begin {
        Write-Verbose "Cmdlet Remove-SiteLanguage - Begin"
    }

    process {
        Write-Verbose "Cmdlet Remove-SiteLanguage - Process"

        $Site = Get-Item -Path $Site.Paths.Path -Language $Language

        Get-ChildItem -Path $Site.Paths.Path -Recurse | ForEach-Object {
            Remove-ItemLanguage -Item $_ -Language $Language
        }
    }

    end {
        Write-Verbose "Cmdlet Remove-SiteLanguage - End"
    }
}

Show-RemoveSiteLanguageDialog.ps1

# Content Editor/Context Menu/Show-RemoveSiteLanguageDialog
# Use a Show Rule to only show for the Site Item template {6669DC16-F106-44B5-96BE-7A31AE82B5B5}

function Show-RemoveSiteLanguageDialog {
    [CmdletBinding()]
    param(
        [Parameter(Mandatory = $true, Position = 0)]
        [Item]$Site
    )

    begin {
        Write-Verbose "Cmdlet Show-RemoveSiteLanguageDialog - Begin"
    }

    process {
        Write-Verbose "Cmdlet Show-RemoveSiteLanguageDialog - Process"

        $languages = [ordered]@{}

        $Site.Languages | ? { $Site.Versions.GetLatestVersion($_).Versions.Count -gt 0 } | ForEach-Object {
            $displayName = [Sitecore.Globalization.Language]::GetDisplayName($_.CultureInfo)
            $languages[$displayName] = $_.Name
        } | Out-Null

        $result = Read-Variable -Parameters `
        @{ Name = "language"; Options = $languages; Value = "en"; Title = "Existing lanuage"; } `
            -Description "This script removes a language from an existing site" `
            -Title "Remove language from the site" -Width 500 -Height 600 `
            -OkButtonName $([Sitecore.Globalization.Translate]::Text("Ok")) -CancelButtonName $([Sitecore.Globalization.Translate]::Text("Cancel")) `

        if ($result -ne "ok") {
            Close-Window
            Exit
        }

        @{
            Language  = $language
        }
    }

    end {
        Write-Verbose "Cmdlet Show-RemoveSiteLanguageDialog - End"
    }
}

Brilliant!