Two of my DC have used Projects Abroad www.projects-abroad.co.uk/?mh_keyword=projects%20abroad&gclid=Cj0KEQiAs6GjBRCy2My09an6uNIBEiQANfY4zBr5oDFd9uMk5yic9Jz-VZne1U993M0QOe21-QDCdMUaAk-g8P8HAQ
DS went to South Africa and worked on a journalism project, producing a glossy magazine about political/environmental issues. Whilst this perhaps strayed close to 'vanity publishing' (it didn't have a cover price and wasn't sold, but was distributed to local unis etc) it was professionally run and he learnt some useful skills, as well as having a good introduction to local SA life and politics, with lots of events to introduce him to how SAfricans manage daily life.
DD2 went to Thailand and taught English in a primary school each morning for a month. She had a reasonable level of training (she was the assistant, so responsible for hearing children read/explaining basic concepts, not teaching a whole class).
Upside: We've found Projects Abroad to be well-organised, briefs students in depth about what is expected of them/cultural differences etc, and good support on the ground in the country they are working in. Also quite flexible on dates/start times and helpful with organising additional travel opportunities once in the country. Both DC stayed with host families - DS's host 'mum' was great with him as a grandmother who had loads of different generations of her family staying with her at various times; DD2's 'mum' was really doing it for the extra cash and less homely atmosphere.
In both DC's cases, the groups of volunteers came from lots of different countries and DD2 wound up learning a fair bit of French as was very friendly with a French business student sent to Thailand as part of her degree and a couple of others.
Downside: Not cheap. Not wholly convinced about the 'worthwhile' element, as effectively in both cases the work was created to suit the volunteers from abroad, rather than being something that really needed doing. DS worked hard at journalism, so probably would describe his experience as a 'working holiday'; for DD2 it was really mostly about the partying!
Friend's DD used Latitude and volunteered on a long-term project in Ecaudor www.lattitude.org.uk/ In some respects this was more worthwhile (was a teaching job at local village school linked to creating work opportunities in tourism for local people) but again difficult to assess. One issue here was that the regular arrival of lots of well-heeled overseas girls who stayed briefly had an adverse impact on relations between local young men and women (basically lots of flirting/affairs; the foreign girls inevitably promised to try and get the young man a ticket to Europe, then went back and forgot about him............)
I may sound cynical, but I think that quite a lot of these gap year projects are fantastic for the volunteer (both my DC learnt a lot about themselves, new skills and how to cope, plus lots of interest in a different country etc) but it is much less clear how worthwhile they really are. (If you Google, there's lots of comment about this at the moment).
So if your French girl's motivation is purely to make a difference, she might need to look very closely at the small print of some of the offerings