create review thru API in specific language

Site Reviews does not currently support translatable reviews.

But, it does support multilingual pages which have reviews assigned to them.

For example, if you have a multilingual Reviews page that you are assigning reviews to:

/en/reviews/
/fr/reviews/
/de/reviews/

And you are using the following shortcodes on those pages:

[site_reviews_summary assigned_posts="post_id"]
[site_reviews assigned_posts="post_id"]
[site_reviews_form assigned_posts="post_id"]

Then by default, each of those pages will only display the reviews that were were assigned to them (i.e. The English page will only display the English reviews, the French page will only display the French reviews, and the German page will only display the German reviews); and will display a rating summary of only the reviews assigned to them (i.e. the English page will only display the rating summary of the English reviews, the French page will only display the rating summary of the French reviews, and the German page will only display the rating summary of the the German reviews).

If you enable the multilingual integration, then each page will display ALL the reviews that were submitted on each language of the page (i.e. The English page will display the English, French, and German reviews, the French page will display the English, French, and German reviews, etc.); and will display a rating summary of the reviews assigned to each language of the page (i.e. the English page will display the rating summary of the English, French, and German reviews, etc.).

So to answer your question:

To submit a French review with the API, make sure that the assigned_posts option in the request points to the Post ID of the French page.