Switch language upon clicking on language menu item

I don't think the current system for storing the preferred language is a good one, but otherwise we'd have to implement putting drop-down preferences in Markdown, which is quite a hassle.

CuraSettingsGuide.py

@@ -406,4 +406,13 @@ def language_list(self, article_key: str) -> List[str]:
 		"""
 		if article_key not in self.article_locations:
 			return []  # We have no articles about this setting at all.
-		return list(self.article_locations[article_key].keys())
+		return list(self.article_locations[article_key].keys())
+
+	@pyqtSlot(str)
+	def set_language(self, language_code: str) -> None:
+		"""
+		Changes the viewing language.
+		:param language_code: The new language code.
+		"""
+		self._selected_language = language_code
+		self.selectedArticleChanged.emit()

resources/qml/TranslationButton.qml

@@ -34,6 +34,7 @@ MouseArea {
 
 			MenuItem {
 				text: code_to_language(modelData.toString())
+				onTriggered: manager.set_language(modelData.toString())
 			}
 			onObjectAdded: languagesMenu.insertItem(index, object)
 			onObjectRemoved: languagesMenu.removeItem(object)