Persist language choice in the preferences

So now there's just no field for it in the preferences article. Later, maybe. It's easy enough to switch anyway.

CuraSettingsGuide.py

@@ -65,14 +65,14 @@ def __init__(self, parent=None) -> None:
 
 		# Add context menu item to the settings list to open the guide for that setting.
 		application = CuraApplication.getInstance()
-		self._selected_language = application.getPreferences().getValue("general/language")
 		application.getCuraAPI().interface.settings.addContextMenuItem({
 			"name": "Settings Guide",
 			"icon_name": "help-contents",
 			"actions": ["sidebarMenuItemOnClickHandler"],
 			"menu_item": MenuItemHandler.MenuItemHandler(self)
 		})
 
+		application.getPreferences().addPreference("settings_guide/language", "cura_default")
 		application.getPreferences().addPreference("settings_guide/show+articles+in+setting+tooltips+%28requires+restart%29", False)
 
 		self.adjust_theme()
@@ -395,7 +395,11 @@ def selectedArticle(self) -> List[List[str]]:
 		items, some of which are text and some of which are image lists.
 		:return: The the currently selected article.
 		"""
-		return self._getArticle(self._selected_article_id, self._selected_language)
+		preferences = CuraApplication.getInstance().getPreferences()
+		language = preferences.getValue("settings_guide/language")
+		if language == "cura_default":
+			language = preferences.getValue("general/language")
+		return self._getArticle(self._selected_article_id, language)
 
 	@pyqtSlot(str, result="QVariantList")
 	def language_list(self, article_key: str) -> List[str]:
@@ -414,5 +418,5 @@ def set_language(self, language_code: str) -> None:
 		Changes the viewing language.
 		:param language_code: The new language code.
 		"""
-		self._selected_language = language_code
+		CuraApplication.getInstance().getPreferences().setValue("settings_guide/language", language_code)
 		self.selectedArticleChanged.emit()