plz allowing users to specify custom instances for each supported translator. Currently, users are limited to using a single instance for all translations, such as `mozhi.example.com`. the idea seeks to provide users with the flexibility to choose different instances for each translator, including Google, Yandex, DuckDuckGo, and LibreTranslate.

**Benefits**

By introducing customizable instances, you can improve the overall user experience in several ways:

1. **Personalization**: Users can tailor the translation process to their preferences by selecting a specific instance for each translator.
2. **Private Translations**: With the ability to specify a custom instance for DuckDuckGo, users can enjoy private translations without relying on mozhi public instances.
3. **Reduced Load on Public Instances**: By using custom instances, we can reduce the load on mozhi public instances, servers, ensuring faster and more reliable translations.

**Implementation**

To implement this feature, we propose the following:

1. Introduce a new configuration option that allows users to specify custom instances for each translator so the user have more options.

**Real-World Use Cases**

This feature can be particularly useful in various scenarios:

* Like someone want to use LibreTranslate with Crow offline all he need to just choice the instance to libretranslate  (e.g., `127.0.0.1:5000` or `127.0.0.1:5010`) ONLY FOR LIBRETRANSLATE so he can still use mozhi for other translators.
* User want to use instance for DuckDuckGo, users can enjoy private translations without relying on mozhi public instances for DuckDuckGo.

By introducing customizable translator instances, I believe Crow Translator can become an even more powerful and user-friendly tool, providing a better experience for its community.