Every FR Engine language consists of one or more base languages. To get the list of all the characters in any FR Engine language, get the characters in all of its base languages.

FREngine.ITextLanguage textLanguage = engineLoader.Engine.PredefinedLanguages.Find("English").TextLanguage;
 System.Text.StringBuilder result = new System.Text.StringBuilder(); 
 foreach (FREngine.IBaseLanguage baseLanguage in textLanguage.BaseLanguages)
 {
 result.AppendFormat("Base language.Alphabet = {0}\n", baseLanguage.LetterSet[FREngine.BaseLanguageLetterSetEnum.BLLS_Alphabet]);
 result.AppendFormat("Base language.Prefixes = {0}\n", baseLanguage.LetterSet[FREngine.BaseLanguageLetterSetEnum.BLLS_Prefixes]);
 result.AppendFormat("Base language.Suffixes = {0}\n", baseLanguage.LetterSet[FREngine.BaseLanguageLetterSetEnum.BLLS_Suffixes]);
 result.AppendFormat("Base language.SubscriptAlphabet = {0}\n", baseLanguage.LetterSet[FREngine.BaseLanguageLetterSetEnum.BLLS_SubscriptAlphabet]);
 result.AppendFormat("Base language.SuperscriptAlphabet = {0}\n", baseLanguage.LetterSet[FREngine.BaseLanguageLetterSetEnum.BLLS_SuperscriptAlphabet]);
 }
 result.AppendFormat("Text language.InterwordPunctuators {0}", textLanguage.LetterSet[FREngine.TextLanguageLetterSetEnum.TLLS_InterwordPunctuators]);
 result.AppendFormat("Text language.Prefixes {0}", textLanguage.LetterSet[FREngine.TextLanguageLetterSetEnum.TLLS_Prefixes]);
 result.AppendFormat("Text language.Suffixes {0}", textLanguage.LetterSet[FREngine.TextLanguageLetterSetEnum.TLLS_Suffixes]);

MessageBox.Show(result.ToString());

Sample output for the English language:

Base language.Alphabet = '-.ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz’
Base language.Prefixes = "'(-.[{©—‘’“•■□▲△►▻▼▽◄◅◊◎◦★☆♦✓❖
Base language.Suffixes = !"')*,-.:;?]}©®—’”™
Base language.SubscriptAlphabet = 
Base language.SuperscriptAlphabet = 

Text language.InterwordPunctuators !"#$%&'()*+,-./:;<=>?[]_{}£¥§©«°»—’“”„•€■□▲△►▻▼▽◄◅◊◎◦★☆♦✓❖
Text language.Prefixes 
Text language.Suffixes