In theory you should just be able to do this:
$item = Get-Item -Path "master:\sitecore\content\sites\abc"
$item.Image = Get-Item -Path "master:\sitecore\media library\xyz\pqr\mno\infotumbnail410PD1"
from this page: https://doc.sitecorepowershell.com/working-with-items
But I couldn't figure out how to do that with fields with a space in them. So I did it ...
When testing your script, I was having an error with the cast of ImageField and also saw that in RAW values, it was adding the value Sitecore.Data.Items.Item. I did a modification of your script and tested it. Please use the below script. You may need to update it to fit your requirements
$thumbnailImage = Get-Item -Path 'master:\sitecore\media library\xyz\...
The path to EditorPage.aspx is hard-coded in Sitecore.Shell.Applications.ContentEditor.RichText. That is the class resposible for the Rich Text field type. You would need to provide an alternate field type implementation if you wanted to point the field at a different aspx page. I wouldn't recommend it.
For most editor customizations, a better approach is ...
You can simply add below code in your layout which will check if the context language is other then FR & EN and will redirect on the Error page.
string lang = Sitecore.Context.Language.ToString();
if(!lang.Contains("en") && !lang.Contains(""fr))
Response.Redirect("Your error page URL");
Hope this helps.