I have locale LANG=en_US.UTF-8 but some tools in busubox all versions doesn't work with unicode correct. For example awk : 

$ echo тест | busybox awk '{ print length($0) }' 
8
$ echo test | busybox awk '{ print length($0) }' 
4
$ echo тест | awk '{ print length($0) }' #it's standart awk of distrib 
4 

same situation is in sed: 
$ echo -ne "тест" | sed -e :a -e "s/^.\{0,10\}$/& /;ta" | wc -m 
11
$ echo -ne "тест" | busybox  sed -e :a -e "s/^.\{0,10\}$/& /;ta" | wc -m 
7
$ echo -ne "test" | busybox sed -e :a -e "s/^.\{0,10\}$/& /;ta" | wc -m 
11