Plone.app.i18n

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Fixes:

- Use zope.interface decorator.
[gforcada]

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Fixes:

- Replace deprecated ``zope.testing.doctest`` and ``zope.testing.doctestunit``
imports with ``doctest`` module from stdlib.
[thet]

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- Fix switchLanguage links. Refs https://github.com/plone/Products.CMFPlone/issues/431
Needs 'use_cookie_negotiation' to be on to actually do anything.
[jaroel]

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- Adapted language viewlet to Plone 5
[bloodbare]

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- Fix url to switchLanguage to include the view
[Gagaro]

- Ported to plone.app.testing
[tomgross]

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