The support of tone and writing_style for French, Italian, Portuguese and Spanish is already present in your DeepL Write Web Interface
https://www.deepl.com/en/write

but it is still missing for the Rephrase feature in the DeepL API Pro (only English UK, English US and German are actually supported).
It would be very helfpul if you could implement it soon.
I mean there that no text improvement will be delivered for French, Italian, Portuguese and Spanish because the parameter tone or writing_style are only supported for English UK, English US and German.
Then you will only get the input sentence wihout any improvement.
If you send your request with 'tone'='friendly' or any allowed value (the same for the parameter 'writing_style', you get then the following error message:
{
"message": "Bad request. Reason: Language Spanish does not support setting a tone"
}
On your Deepl Write Web interface those languages ( French, Italian, Portuguese and Spanish) are fully supported and due to this I sent my request.
Best regards.
Regis Barbier