Ttgjyuk

i uses count function instead of showing "ı" character.
Tried i, i $$i$$, imath texti but continues to count.

documentclass[12pt, letterpaper, oneside]article

usepackage[T1]fontenc
usepackage[turkish]babel
usepackage[utf8]inputenc
usepackagedatetime
usepackageamsmath
usepackagefancyhdr
usepackagegraphicx
usepackage[colorinlistoftodos]todonotes
usepackagetabularx
usepackageurl
usepackagebooktabs
usepackagetitlesec
usepackagefloat
usepackagehyperref
usepackagehypcap
usepackageapacite
usepackageneedspace %sayfa sonu boşluklarını ayarlamak için
usepackage[section]placeins 
usepackagexcolor
hypersetup 
 colorlinks,
 linkcolor=red!50!black,
 citecolor=blue!50!black,
 urlcolor=blue!80!black

bibliographystyleapacite 
usepackagefancyref
usepackagetikz
usetikzlibrarybabel
usepackagepgf-pie %piechart için
usepackagesubcaption
usepackagepgfplots
usepackagelongtable
usepackagemakecell
usepackagebooktabs
usepackagelscape

usepackage[font=footnotesize]caption
newcommandsekil[1](Şekil reffig:#1)
newcommandtablo[1](Tablo reftab:#1)
newcommandHRulerulelinewidth0.5mm
usepackagelmodern
usepackageetoolbox
AtBeginEnvironmenttikzpictureshorthandoff;

%---------------------- Header -------------------------------%
fancyhead[L]R. Özgür ALTUN
fancyhead[R]Tangonun Sosyal Dansçıları
fancyhead[C]
fancyfoot[C]thepage 
pagestylefancy
setlengthheadheight26pt


begindocument
beginfigure[htb!]
begincenter
begintikzpicture[scale = 0.9]
inputencodingutf8
usetikzlibrarybabel
 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 akicii yapmak, 2/Keyif almak, 51/Mükemmel şekilde yapana kadar tekrar etmiş olmak
endtikzpicture
captionYeni figür uygulanmasında öncelikli tercih. 
labelfig:yenifiguroncelik
endcenter
endfigure

enddocument

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

lettikzi=i % Fix for bad programming in pgf-pie

to your document preamble, just before begindocument, 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.

begintikzpicture[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 akicitikzi yapmak,
 2/Keyif almak,
 51/Mükemmel şekilde yapana kadar tekrar etmiş olmak

endtikzpicture

Found an easy solution at second hour of searching. symbol25 worked for me.