Zenzizenzizenzic

- "As well as being a mathematical oddity, it survives as a linguistic oddity: zenzizenzizenzic has more Zs than any other word in the OED.[5][6]"

It's not even close:

    $ cat /usr/share/dict/words | tr 'A-Z' 'a-z' | tr -dc 'z\n' | sort | uniq -c

    628483 
     24913 z
      1485 zz
        37 zzz
        18 zzzz

Next closest I could find, in any language, is 5 'z's for "zbezczeszczać", or 6 'z's for its inflected form "zbezczeszczasz".

https://en.wiktionary.org/wiki/zbezczeszczać#Polish