In general SWs don't look for an exact mix of ethnicity, but they do want families that can 'promote a positive sense of identity' - or similar in SW speak!
There are all sorts of factors involved in that.
Some things are not specific to whether the child's heritage is from e.g. a particular country or region. Something as simple as looking as if they could be the biological child of the parents cuts out a huge amount of intrusive questioning. Having first hand experience of the everyday expressions of racism means you would be more able to support your child when they have similar experiences.
The specifics can be met with a bit of willingness to make an effort. Familiarity with the child's culture of origin helps them to understand and value that culture - but you could also learn about the culture and find a way of integrating it into your broader family culture.
DD is black, I'm white (we're not in the UK). The biggest issue for us (so far) is that we look so different and all too many people seem to think it's OK to ask/ speculate about why. I've made a point of choosing housing, school, church etc to be diverse and reflecting both our heritages, and we know a number of other families who have adopted trans-racially.
The next challenge is to learn her first language (which she has since forgotten, but can still pronounce), I can speak a little but not really sustain a conversation. Our church is starting classes, so we'll go to those.
So, I doubt the ethnic mix in your family would be a problem, probably quite the opposite.
Re: the language, DD spoke very little English at 5 when I adopted her (she had learnt a bit at pre-school). Within a few months you wouldn't have known. It wasn't ideal - and she does have some mild speech/ language difficulties which may be connected to that, but they are the sorts of difficulties that are quite common in children who have experienced chaotic early years and don't switch language, so who knows. You might need to start by speaking English and gradually introduce French, but again, as long as you had a child-sensitive plan I doubt it would be a deal breaker.