Yes it is best to know the local language and you can get by here on a hundred Malagasy words that can cover all the dialects.
You do need to know some French as in the numbers one to hundred and one thousand, as these numbers are used in counting money.Â
I visited Brazil in the 1980's and got by on English but I think they also spoke some French as well, so you may already know the numbers.Â
If I come across something more to your question I will post it here or in a Private message.