After I updated Debian Stretch (9) to Debian Buster (10) I noticed that the locales without ending UTF8 extension we're no longer provided.
Instead of patching every application to use the locale with ending UTF8 extension I decided to generate the missing locales by applying the following steps:
Add missing locales to the locale.gen file
vi /etc/locale.gen # Add to the bottom: nl_BE UTF-8 nl_NL UTF-8 en_GB UTF-8 de_DE UTF-8 es_ES UTF-8 fi_FI UTF-8 fr_FR UTF-8 it_IT UTF-8 # Generate locales: locale-gen
After the generation is completed restart your application, in my case PHP.