Make Culture lookup case-insensitive; add GetOrDefault()

Changed Culture indexer to use case-insensitive language matching.
Added GetOrDefault() to return a Culture or the default if not found.

EnvelopeGenerator.Web/Models/Cultures.cs

@@ -7,7 +7,9 @@
         public IEnumerable<string> FIClasses => this.Select(c => c.FIClass);
 
         public Culture Default => this.First();
-        
-        public Culture? this[string? language] => language is null ? null : this.Where(c => c.Language == language).FirstOrDefault();
+    
+        public Culture GetOrDefault(string? language) => this[language] ?? Default;
+
+        public Culture? this[string? language] => language is null ? null : this.FirstOrDefault(c => string.Equals(c.Language, language, StringComparison.OrdinalIgnoreCase));
     }
 }