Go to http://recommend.wmflabs.org/ -
choose a source language as Russian or any language with special characters - ie- when there is URL encoding used
choose English as the target language
Click on a recommended page with special characters in title - it fails to load on the next page and the interface cannot proceed further
Click on a recommended page which only has regular a-z,A-Z,0-9 characters and the page loads fine
Instead of going via the recommend page, go to the same source article - for instance the German article
Click on the English interwiki link if preferences are set up and the translate tools work fine from there.
The above recommend page fails presumably due to the URL encoding and decoding involved
My system language is English and I use a Firefox browser (54.0.1).