$ctx = Get-SPOContext
$web = Get-SPOWeb

function AddSupportedUILanguageToWeb($web)
{
    $web.Url

    $web.IsMultilingual = $true
    $web.AddSupportedUILanguage(1043)
    $web.Update()
    $ctx.ExecuteQuery()
}

AddSupportedUILanguageToWeb $web

$subWebs = Get-SPOSubWebs -Recurse

foreach($subWeb in $subWebs)
{
    AddSupportedUILanguageToWeb $subWeb
}