How to change text encoding when inserting values inside a sharepoint list using powershell?

I'm using french words with special characters like "é" "à" etc. When I trying to insert text in my list fields using powershell, it does insert the values but the characters aren't well encoded. For exemple I have a "è" instead of having "è".

    $weburl = "http://mysite/"
    $web = Get-SPWeb $webUrl
    $list = $web.GetList("/Lists/myList/")
    $newItem = $list.AddItem()
    $newItem["Titre"] = "Problème" # <-- here is the issue
    $newItem.update()

How can I fix this?