ULS should have an option to disable language detection for anonymous users, in a way that
a) applies no language detection and instead uses the content language
c) initially does not set (and if necessary, deletes) any ULS cookies
b) but still shows the ULS widget to anonymous users
d) sets an ULS cookie if and when the user selected a language manually using the ULS widget (or by using the setlang=xx URL parameter).
This was suggested by Erik Möller as a solution to bug 41451, see comment 47 there. It allows ULS to be used an environment such as the Wikimedia cluster, where web caches are used, and these caches can be configured to not cache responses to queries the contain specific cookies.
This is a nicer, but less robust, approach than bug 42157.