You have 0000016125 in there which, while not technically illegal (it is a Text node), is just kind of odd.
Your element contains invalid characters without an XML CDATA section
You can manually normalize the XML or do it in C# via string manipulation, or RegEx, or something similar.
In your simple example, only the element has invalid characters; therefore it is relatively simple to fix it and add a CDATA section using the string.Replace() method, to make it look like this:
Then you can load the good XML into your XDocument using XDocument.Parse(string xml):
string badXml = @"
UTF160000016125
0003┴300000┴English(U.S.)PORTUGUESE┴┴bla.000┴webgui\messages\xsl\en\blabla\blabla.xlfTo blablablah the firewall to blablablah local IP address. Para blablablah a uma blablablah local específico. ";
// assuming only element has the invalid characters
string goodXml = badXml
.Replace("", "", "]]>");
XDocument xDoc = XDocument.Parse(goodXml);
xDoc.Declaration = new XDeclaration("1.0", "utf-16", "yes");
// do stuff with xDoc