src/vs/workbench/services/editor/browser/editorService.ts/EditorService/activeTextEditorLanguageId
class EditorService:	get activeTextEditorLanguageId(): string | undefined {
		let activeCodeEditor: ICodeEditor | undefined = undefined;

		const activeTextEditorControl = this.activeTextEditorControl;
		if (isDiffEditor(activeTextEditorControl)) {
			activeCodeEditor = activeTextEditorControl.getModifiedEditor();
		} else {
			activeCodeEditor = activeTextEditorControl;
		}

		return activeCodeEditor?.getModel()?.getLanguageId();
	}