Thanks for your answer.
I’ve looked at the blog page link you gave and a few other openframeworks webpages.On a different website I find Turkish characters ı,ğ,ş were supported in Unicode Latin Extended Additional.
I tried:
ofTrueTypeFont setting("ARIALUNI.TTF",24);
setting.addRanges(ofAlphabet::Latin);
or
setting.addRange(ofUnicode::Latin);
setting.addRange(ofUnicode::LatinExtendedAdditional);
setting.addRange(ofUnicode::Latin1Supplement);
gui.loadFont(setting);
gui.add(label1.setup("abcçdegğhıijkl","234",375,250));
label1.loadFont(setting);
I tried similar things and combinations.Just using:
setting.addRange(ofUnicode::LatinExtendedAdditional);
cause no display of text maybe there was/is a problem with ofUnicode::LatinExtendedAdditional.
In the end, I found a thread related to Turkish characters under Openframeworks 0.10.0(the version which I am currently using:
0.10.0 Release candidate 4https://forum.openframeworks.cc/t/0-10-0-release-candidate-4/29423/
There a user named Alp suggests a quick fix for Turkish characters problem,I tried it and it worked 
"
Alp wrote:
May '18
I tried to include unicode characters as follows;
> ofTrueTypeFontSettings settings("verdana.ttf", 14);
> settings.addRanges(ofAlphabet::Latin);
> settings.contours=true;
> settings.antialiased=true;
I can’t display “ğ,Ğ,ı,İ,şŞ”. I’ve just add the following line (LatinA chars range) to ofTrueTypeFont.h
static const range LatinA;
And, to ofTrueTypeFont.cpp
> const ofUnicode::range ofUnicode::LatinA {0x0100,0x017F};
> const std::initializer_list<ofUnicode::range> ofAlphabet::Latin {
> ofUnicode::Latin1Supplement,
> ofUnicode::LatinExtendedAdditional,
> ofUnicode::LatinA
> };
Now, Latin alphabet includes full Turkish language character set.
"
So now using something like:
setting.addRange(ofUnicode::LatinA);
setting.addRange(ofUnicode::Latin1Supplement);
works.
I wonder if ofxaddons now support Unicode too, like ofxImGui or ofxDatGui.