I've been suffering from an inferiority complex ever since reading about that 15-letter Finnish palindrome for a soap-dealer, SAIPPUAKAUPPIAS. Surely the English language can come up with something longer than the 7-letter ROTATOR, REPAPER or DEIFIED. (Yes, I know of the 9-letter MALAYALAM and the 11-letter KINNIKINNIK variant, but these are both borrowed words from non-European languages.)
Ulrich, Eugene J.
"Soap Dealers and Beer Barons,"
Word Ways: Vol. 15
, Article 7.
Available at: https://digitalcommons.butler.edu/wordways/vol15/iss4/7