You might want to use the [xml]
type.
# Cast the xml string into [xml] type
[xml]$xml = @"
<?xml version="1.0" encoding="utf-8"?>
<Book>
<projects>
<project name="Book1" date="2009-01-20">
<editions>
<edition language="English">En.Book1.com</edition>
<edition language="German">Ge.Book1.Com</edition>
<edition language="French">Fr.Book1.com</edition>
<edition language="Polish">Pl.Book1.com</edition>
</editions>
</project>
</projects>
</Book>
"@
$xml.SelectNodes("//edition")
Outputs
language #text
-------- -----
English En.Book1.com
German Ge.Book1.Com
French Fr.Book1.com
Polish Pl.Book1.com
And for your case specifically,
$Xml.SelectNodes("//edition")."#text"
Returns
En.Book1.com
Ge.Book1.Com
Fr.Book1.com
Pl.Book1.com