Also need to lose the quotes around $id 
"/language/$language/widget[\@ID=$id]"

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Update:
Thanks to choroba's explanations and examples below, this only works by accident, i.e. only if $id is a number.

Here are some different ways of doing it: 

"/language/$language/widget[\@ID='$id']"
"/language/$language/widget[attribute::ID='$id']"
qq|/language/$language/widget[\@ID="$id"]|
qq|/language/$language/widget[\@ID='$id']|
qq|/language/$language/widget[attribute::ID="$id"]|
qq|/language/$language/widget[attribute::ID='$id']|

[download]

In reply to Re^2: language selection via libxml by tangent
in thread language selection via libxml by banzai

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