SP-API listing on foreign language marketplace, auto translation of description and bullet points

Let’s say I am a UK seller and I have all of my items listed on GB marketplace. When I list my items my description and bullet points are in English and I set the language_tag to en_GB when I set them. No problems here.

I list them on US marketplace and I use English descriptions and bullet points and set language tag to en_US. No problems here too.

Now let’s say I want to list on the Amazon FR marketplace. If I manually translate my description and bullet points to French and set the language_tag to fr_FR, everything is fine, but if I use my English descriptions and bullet points and set the language_tag to en_GB on the french marketplace, the description and bullet points are ignored. It was my understanding that Amazon should automatically translate description and bullet points into the marketplace language if they are provided with an alternative language_tag.

Anyone have any experience of this. All of my France marketplace items are sat there with no description and no bullet points, and never get the buy box because of this.

If I call getListing it shows the items have an English description set in en_GB language tag, but this is never used or translated.