How to write ı (i without dot) character in pgf-pie

I'm afraid there's little to do. The package pgf-pie is very badly written.

While in a user document one can choose to use \i inside a \foreach cycle, this should not be inflicted to the users of a package, which should be as neutral as possible.

Ask the developers to change the local variables in their \foreach cycles.

In the meantime, add

\let\tikzi=\i % Fix for bad programming in pgf-pie

to your document preamble, just before \begin{document}, and in the text for a pie chart use \tikzi where \i should be used. No direct UTF-8 input, but the explicit command \tikzi.

\begin{tikzpicture}[scale = 0.9]
  \pie[
    sum=auto,
    text=legend,
    color={black!0, black!10, black!30, black!50, black!80},
    explode=0.1,
    scale font,
    before number= {\%}
  ]{%
    9/Doğru veya yanliş olduğunu umursamadan keyif almak,
    11/Çalişilan bir eş ile yapmak,
    26/Hatali da olsa akici\tikzi{} yapmak,
    2/Keyif almak,
    51/Mükemmel şekilde yapana kadar tekrar etmiş olmak
}
\end{tikzpicture}