DrawString() don't have any problem while writing special characters if they are provided in font file (.ttf)
Problem can be in the following line
System.Drawing.Text.PrivateFontCollection fontCollection = new System.Drawing.Text.PrivateFontCollection();
Specially with the font you get in ff1
FontFamily ff1 = fontCollection.Families[0];
In your case it means Font ff1 don't have representation for some or all special characters.
Since I don't know how many font files you are getting from fontCollection.Families, if fontCollection.Families contain more than one font file try next one.
FontFamily ff1 = fontCollection.Families[1]; // and so on.
Or try to add font file with all special characters (if System.Drawing.Text.PrivateFontCollection is absolutely necessary ).
Alternatively you can use InstalledFontCollection instead of PrivateFontCollection
System.Drawing.Text.InstalledFontCollection fontCollection = new System.Drawing.Text.InstalledFontCollection();
Which will give you all installed fonts of the system on which application is running through its Families property InstalledFontCollection.
Easiest way is (if only one font is required, as you are using only one font in your provided code) using constructor of FontFamily class with a string type parameter FontFamily.
Consider replacing this line
FontFamily ff1 = fontCollection.Families[0];
With this one
FontFamily fontFamily1 = new FontFamily("Arial");
Arial must be installed on the system where application is running.
Hope this will help.