Expose spell language setting in UI (or make more discoverable)

### Problem

The default spell language is `en_US`. There does not seem to be a way of changing it through the user interface.

### Proposal

When the plugin is active it should add an option to a context menu, the appmenu or otherwise expose the setting in the interface.

### Prior Art (Optional)

_No response_

未指定文件、测试或入口。首先定位拼写检查插件和现有的拼写语言设置，然后检查其他设置是如何在界面中提供的；当用户能够通过 UI 发现并更改拼写语言时，即视为完成。

由索引模型根据 Issue 内容生成。