[Lazarus] substr return wrong string with some utf8 char

[claude Pomalo]

hello

i use the substr function to get some truncated string.
these string are in french language  and sometimes contains "à" or "é" 
etc.. char
When these charaters are in the string the wrong substr is return
substr (pierre à feu,11) return 'pierre à  f' not 'pierre à fr'

length('à') return 2
utf8length('à') return 1

is a utf8Subst() like function exist?

thank
Claude