Yes, write in French.
BTW, before getting my contract with Ulys, I did try Mango.
At the time I had the DD/Sepa issue (this is before I started using Wise as the payment account) and Mango wasn't able to set up the DD on their side after I paid for the blipper to be shipped (by card).
They got kinda aggressive because they couldn't set up the payment (their fucking problem because they didn't test their service, clearly) while I spent quite a lot of time to try and sort it out with HSBC my side.
After a couple of weeks they sent a message announcing that the were closing my account (which they did) and ordered me to return the blipper or legal action blah blah.
I binned the blipper and nothing ever happened.
I then tried with Ulys and, after a few weeks and some arguing I managed to make it work with Wise (and leaving a 40 quid deposit for my first blipper).
It works now, I'm happy, but it was a faff.
Most of it is, and I repeat, because they didn't test their service. Surely they would have noticed that they are not able to set up SEPA payments for 90% of their users.
Also, having English speaking personnel for a service aimed at British users could help sometimes. But I guess it is difficult to predict that people in the UK do speak English mostly and not French.
I'm kinda used to poorly designed digital (and not) services in Italy, but the French do take the cake most times in my experience

Or maybe they are using the same people who build Garmin apps and software.
I wonder how much their conversion rates would improve if they even just explained or suggested to UK users what we discussed at length on these threads here.
Long story short: use google translate (both ways) for any support communication.