different languages have different rules for:
which index terms should go into the same group
e.g. in Czech words starting with letters „c“ and „č“ go to separate groups, but words starting both with „a“ and „á“ go to the same group
how the terms are alphabetically sorted
e.g. in Czech there is special two character letter „ch“. This letter is ordered between „h“ and „i“. Similar letters are in traditional Spanish or Hungarian.
some languages combine previously mentioned behavior together
e.g. in German letter „ö“ is placed into the same group as „o“, but it is sorted as if it were two letter sequence „oe“
things are getting complicated and we even did not start talking about CJKV languages