If Plesk throws an error that it can't upgrade your license key because of languages, you need to remove the extra locales:
# rpm -qa | grep psa-locale | grep -v base
psa-locale-el-GR-8.1-build81061127.19
psa-locale-fr-FR-8.1-build81061127.19
psa-locale-lt-LT-8.1-build81061127.19
psa-locale-pt-BR-8.1-build81061127.19
psa-locale-sv-SE-8.1-build81061127.19
psa-locale-ca-ES-8.1-build81061127.19
psa-locale-de-DE-8.1-build81061127.19
psa-locale-es-ES-8.1-build81061127.19
psa-locale-fi-FI-8.1-build81061127.19
psa-locale-hu-HU-8.1-build81061127.19
psa-locale-ja-JP-8.1-build81061127.19
psa-locale-nl-BE-8.1-build81061127.19
psa-locale-pl-PL-8.1-build81061127.19
psa-locale-pt-PT-8.1-build81061127.19
psa-locale-ru-RU-8.1-build81061127.19
psa-locale-tr-TR-8.1-build81061127.19
psa-locale-zh-TW-8.1-build81061127.19
psa-locale-cs-CZ-8.1-build81061127.19
psa-locale-es-MX-8.1-build81061127.19
psa-locale-it-IT-8.1-build81061127.19
psa-locale-nl-NL-8.1-build81061127.19
psa-locale-ro-RO-8.1-build81061127.19
psa-locale-zh-CN-8.1-build81061127.19
# rpm -ev `rpm -qa | grep psa-locale | grep -v base`
