Not sure about this but would it be worth you giving Clicker a ring as they do MFL packs?  Not sure how compatible these packs would be with the access method your pupils is using...is s/he using a switch or eye gaze or something else?  Also might be worth seeing what the local special schools offer in the way of MFL.  I used to be an MFL teacher so I always wanted it to be for everyone, but some schools just disapply anyone with a difficulty in this area so I haven't had many pupils go up to this level with MFL.

There is also some info here:  https://www.assistiveware.com/innovations/bilingual-aac  but this seems to be more for actual bilingual users - they might be able to help if you contact them though.  Good luck - I would be interested to hear if you get anywhere with this!

There may be some barriers here when it comes to GCSEs. I am told that, for instance, if a candidate has a human scribe in an MFL written exam they MUST spell each and every word for the scribe as one of the skills being assessed is the spelling of the text. This would seem to be very difficult for an AAC user and care would need to be taken to ensure predictive text was not available, and that they were not selecting pre-programmed words from their device. For a spoken exam, what is being assessed is the knowledge of the language, choice of vocabulary, correct verbs conjugated, tenses etc. and accent. I suspect that this would mean this exam cannot be adapted to allow AC access without damaging the integrity of the assessment and as such there is no possible reasonable adjustment.  My exams officer is checking with the exam board to see if rules have changed and will update this if that is the case.

Currently I advise pupils here that learning to understand and communicate in additional languages is very worthwhile but that carrying this on to GCSE is not advisable due to the skills being assessed.

Further to my comment above, feedback from the exam board today:

Listening, writing and reading exams - fine to use AAC to answer.  AAC users would NOT have to spell every word where a pupil with a broken arm would still have to do so.

Speaking exam - probably no way for access so would be exempt from this section and graded pro-rata on the remaining elements.

Thank you for that information. I taught French before moving in to SEND. When I was working an an Advisory Support Teacher for pupils with Physical Medical needs it always annoyed me that pupils were withdrawn from MFL. The exam board answer is very positive.