Write Brief: assistant does not detect site language

### Impacted plugin

Jetpack

### Quick summary

Brieve provides recommendations in English, even though my site language is set to French.

### Steps to reproduce

1. Start with a site where the site language is set to French.
2. Install the Jetpack plugin, and connect it to WordPress.com.
3. Go to Posts > Add New
4. Start writing a few lines in French.

### A clear and concise description of what you expected to happen.

I expect Brieve to give me recommendations for the few lines I wrote.

### What actually happened

It highlights many orthographic mistakes, because it does not recognize that I'm writing in French.


### Impact

Some (< 50%)

### Available workarounds?

No but the platform is still usable

### If the above answer is "Yes...", outline the workaround.

_No response_

### Platform (Simple and/or Atomic)

Self-hosted

### Logs or notes

_No response_