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-genAfter the generation is completed restart your application, in my case PHP.