Change language tag of input fields to standard to enable spell checking (#140)

* change language tag from data-lang to lang to enable spell checking

See https://www.w3.org/International/questions/qa-html-language-declarations for reference

* change language tag from data-lang to lang in correspondence to i18n.py

change necessary to enable spell checking in textareas

src/pretix/base/i18n.py

@@ -121,7 +121,7 @@ class I18nWidget(forms.MultiWidget):
         self.field = field
         for lng in self.langcodes:
             a = copy.copy(attrs) or {}
-            a['data-lang'] = lng
+            a['lang'] = lng
             widgets.append(self.widget(attrs=a))
         super().__init__(widgets, attrs)