Use single available language of country as default language

This way, one does not have to define a default language for a country with only a single available language.
Remove explicit setting of default language for single-language countries

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

Author: lauft

ChangeLog

@@ -1,3 +1,5 @@
+- #146 Use single available language of country as default language
+
 2026.1.1
 - #145 Add how-to guides
 - #144 PL: Add Christmas Eve as holiday

src/holidata/holiday.py

@@ -124,6 +124,8 @@ def build_for_year_and_lang(self, year: int, lang: str) -> Iterator[Holiday]:
                 notes=self.notes,
             )
 
+        return None
+
 
 class Country(metaclass=PluginMount):
     """
@@ -144,6 +146,7 @@ def __init__(self):
         if self.default_lang is not None and self.default_lang not in self.languages:
             raise ValueError(f"Country '{self.__class__.__name__}' does not list language '{self.default_lang}'!")
 
+        self.default_lang = self.languages[0] if self.default_lang is None and len(self.languages) == 1 else self.default_lang
         self.holiday_generators = []
         self.regions = []
 
@@ -157,6 +160,8 @@ def validate_language_or_get_default(self, lang_id: str) -> str:
                 f"Language '{lang_id}' is not defined for country '{self.id}'! Choose one of [{', '.join(self.languages)}].")
         elif lang_id is None and self.default_lang:
             lang_id = self.default_lang
+        elif lang_id is None and len(self.languages) == 1:
+            lang_id = self.languages[0]
         elif lang_id is None:
             raise ValueError(
                 f"Country '{self.id}' has no default language specified! Choose one of [{', '.join(self.languages)}].")

src/holidata/holidays/AT/__init__.py

@@ -11,7 +11,6 @@
 class AT(Country):
     id = "AT"
     languages = ["de"]
-    default_lang = "de"
     easter_type = EASTER_WESTERN
 
     def __init__(self):

src/holidata/holidays/AU/__init__.py

@@ -18,7 +18,6 @@
 class AU(Country):
     id = "AU"
     languages = ["en"]
-    default_lang = "en"
     easter_type = EASTER_WESTERN
 
     def __init__(self):

src/holidata/holidays/BR/__init__.py

@@ -47,7 +47,6 @@
 class BR(Country):
     id = "BR"
     languages = ["pt"]
-    default_lang = "pt"
     easter_type = EASTER_WESTERN
 
     def __init__(self):

src/holidata/holidays/CH/__init__.py

@@ -11,7 +11,6 @@
 class CH(Country):
     id = "CH"
     languages = ["de"]
-    default_lang = "de"
     easter_type = EASTER_WESTERN
 
     def __init__(self):

src/holidata/holidays/CO/__init__.py

@@ -11,7 +11,6 @@
 class CO(Country):
     id = "CO"
     languages = ["es"]
-    default_lang = "es"
     easter_type = EASTER_WESTERN
 
     def __init__(self):

src/holidata/holidays/CZ/__init__.py

@@ -19,7 +19,6 @@
 class CZ(Country):
     id = "CZ"
     languages = ["cs"]
-    default_lang = "cs"
     easter_type = EASTER_WESTERN
 
     def __init__(self):

src/holidata/holidays/DE/__init__.py

@@ -27,7 +27,6 @@
 class DE(Country):
     id = "DE"
     languages = ["de"]
-    default_lang = "de"
     easter_type = EASTER_WESTERN
 
     """

src/holidata/holidays/DK/__init__.py

@@ -11,7 +11,6 @@
 class DK(Country):
     id = "DK"
     languages = ["da"]
-    default_lang = "da"
     easter_type = EASTER_WESTERN
 
     def __init__(self):