C# Basic Label Class ready for Localization

LabelBook.cs

using System.Collections.Generic;
using System.Text.RegularExpressions;
using System.Globalization;

namespace BrainstationShared.Helpers {
  public class LabelBook {
    public Dictionary<string, LanguageBook> Book { get; set; }
    public string SetLang { get; set; } = "en";
    public const string NoLabelPlaceholder = "NOT_SET";

    public LabelBook(Dictionary<string, LanguageBook> book) {
      this.Book = book;
    }

    public LabelBook(Dictionary<string, LanguageBook> book, string lang) {
      this.Book = book;
      this.SetLang = lang;
    }

    public LanguageBook this[string id] {
      get {
        return this.Book.ContainsKey(id) ? this.Book[id] : new LanguageBook() { en = LabelBook.NoLabelPlaceholder };
      }
      set {
        this.Book.Add(id, value);
      }
    }

    public string GetLabel(string id) {
      return this.Book.ContainsKey(id) ? this.Book[id][this.SetLang] : LabelBook.NoLabelPlaceholder;
    }
    public string GetLabel(string id, string lang) {
      return this.Book.ContainsKey(id) ? this.Book[id][string.IsNullOrEmpty(lang) ? this.SetLang : lang] : LabelBook.NoLabelPlaceholder;
    }

    // always in 'en'. camel-cased. Example Text => exampleText
    public string GetHtmlId(string key, bool isTitlecase = false) {
      LanguageBook labelEntry;
      Book.TryGetValue(key, out labelEntry);
      TextInfo tcId = new CultureInfo("en-US", false).TextInfo;
      // change to titlecase
      string titlecasedId = labelEntry != null && !string.IsNullOrEmpty(labelEntry["en"]) ? tcId.ToTitleCase(labelEntry["en"].ToLower()) : null;
      // strip out all symbols except numbers and characters, lowercase the first character
      return titlecasedId != null ? Regex.Replace(isTitlecase ? titlecasedId : char.ToLower(titlecasedId[0]) + titlecasedId.Substring(1), @"[^a-zA-Z0-9]+", "") : null;
    }
  }

  public class LanguageBook { 
    public string en;
    public string fr;
    public const string NoLangPlaceholder = "NOT_SET_IN_LANG";

    public string this[string lang] {
      get {
        string finalStr;
        switch (lang) {
          case "en":
            finalStr = this.en;
            break;
          case "fr":
            finalStr = this.fr;
            break;
          default:
            finalStr = this.en;
            break;
        }

        return finalStr ?? LanguageBook.NoLangPlaceholder;
      }
    }
  }
}