Fix language handling

Language id can be given in upper or lower case in the main script.
Internally, the language id is handled in lower case.

Signed-off-by: Thomas Lauf <thomas.lauf@tngtech.com>

src/holidata/holidays/holidays.py

@@ -214,7 +214,7 @@ def validate_language_or_get_default(self, lang_id):
             raise ValueError(
                 f"Country '{self.id}' has no default language specified! Choose one of [{', '.join(self.languages)}].")
 
-        return lang_id
+        return lang_id.lower()
 
     def define_holiday(self):
         generator = HolidayGenerator(self.id, self.default_lang, self.easter_type)