The Plone 5.2 build is broken (https://jenkins.plone.org/job/plone-5.2-python-2.7/1745/consoleText) because the last release of p.a.multilingual was botched:
5.2.1 was released in June (maybe by @mauritsvanrees) https://pypi.org/project/plone.app.multilingual/5.2.1 but the version-bump seems to not have been pushed back to master (or might have been overwritten).
So the new release (that should have been 5.2.2 is now 5.2.1 did not make it to pypi and 5.2.1 is missing all the code to continue to work in Plone 5.2. Long story short: Please re-release plone.app.multilingual as 5.2.2 including the changes in master. It moslty breaks because https://github.com/plone/plone.app.multilingual/pull/321 is missing.