The behavior is introduced clearly in the description. Just read the description so you wont be confused then. I cannot understand why someone does not have time to read it, but are willing to have a try and leave a even longer comment here. Basically, this is what WebExtension (extensions for Firefox 57+) may done currently. If you prefer a fx24 like search bar, you would be better to switch to other browsers.

It sets default search engine list based on your browser language setting. The default search engines only used to initial your search engine list and fill it when you try to reset your settings. You may simply delete any of them as you like. I had a double check about the initial search engine list. Only two Chinese search engines (and some common search engines in Chinese language, including Amazon and Wikipedia) are included here. If your browser is configured as simplified Chinese, baidu would be included; If your browser is configured as traditional Chinese, readmoo would be included. If not so, it should be a bug. You could report any bugs to me. But please include your browser language setting so I can debug it.

Anyway, this extension is free and open source. You are welcomed to fork it and develop another extension for whatever you want. We would be happy if someone like you would create a better extension for this use case. Only remember that this extension is published under MPL-2.0 if you do so.