Make redlink tooltip be content language to stop cache pollution.

Make redlink tooltip be content language to stop cache pollution.

Redlink tooltips end up in parser cache!

This is not ideal, as ideally it would be in the page's language,
not the content language and it would be user language for things
outside of pages, but that's not easy to do the way the code is
structured, and this is much better than the alternative.

Other possibilities include:
* Splitting parser cache (Previously been reverted, and doesn't
seem worth it given the feature in question)
* Post-processing step after parsing (Really complicated. Doesn't
seem worth it)

Bug: T34686
Change-Id: Id632f8ef59d1c762aed1867a708c569cbff5f0dd