Factor out detection if we can add new language

Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/trans/models/subproject.py

@@ -1487,20 +1487,29 @@ class SubProject(models.Model, PercentMixin, URLMixin, PathMixin):
             )
         return self._all_flags
 
-    def add_new_language(self, language, request):
-        '''
-        Creates new language file.
-        '''
+    def can_add_new_language(self):
+        """Wrapper to check if we can add new language."""
         if self.new_lang != 'add':
-            raise ValueError('Not supported operation!')
+            return False
 
         if not self.file_format_cls.supports_new_language():
-            raise ValueError('Not supported operation!')
+            return False
 
         base_filename = self.get_new_base_filename()
         if not self.file_format_cls.is_valid_base_for_new(base_filename):
+            return False
+
+        return True
+
+    def add_new_language(self, language, request):
+        '''
+        Creates new language file.
+        '''
+        if not self.can_add_new_language():
             raise ValueError('Not supported operation!')
 
+        base_filename = self.get_new_base_filename()
+
         filename = self.file_format_cls.get_language_filename(
             self.filemask,
             language.code