I was there last summer.
You can find out what you need from the French AIP. Have a look at their VAC for Belle Ile. They are well presented.
They can be found here
http://www.sia.aviation-civile.gouv....set_aip_fr.htm
From that chart:
Information relating to dangerous activities in the
vicinity of BELLE ĪLE is available on LANN BIHOUE
APP FREQ. 123.0 (Area D13) or CC MAR Atlantique
call sign ARMOR FREQ. 124.725 MHZ (Area D18).
I spoke to Armor last year and they spoke perfect english. Any enroute ATC or ATC at a controlled airport that I came across, spoke perfect english.
Belle Ile is listed as a AFIS (with english). However when I went there, despite it being a time that AFIS was supposed to be operating, there was none. So it was back to French radion calls. Have a sheet with the French calls with you in case you need them. The airport was quiet enough, so it wasn't much of an issue.
Belle Ile is a lovely place. I suggest you get a hire car though. You will need it to get around. They will deliver it to the airport for you, if you order it in advance. It's also possible to leave it back to the airport, with prior arrangement.
dp