I would disagree about the comment on 'true' Montessori being bilingual, and I am not sure where that information comes from?
In my experiences internationally, one of the reasons why Montessori schools are often bilingual in non-English speaking countries is because almost all the Association Montessori Internationale trainers are english-speaking, and it is the 'common' language across the organisation. Most teachers are trained in English, so when sourcing teachers schools often have to take this into account. Also, offering a bilingual programme is a plus for schools who have to compete for high-fee-paying parents where there is not government subsidy...basically it's 'value-adding'...
However, there is training in Japan, Pakistan, India, Mexico, France, Germany, Switzerland and Italy which are not first-language English...
That said, Montessori does have an effective model for teaching in a bilingual way, and my key thing would be to check that your child's teachers are native/very fluent speakers in whatever language you want...particularly for preschool...
My children have had Montessori teachers trained in Sri-Lanka, Malaysia, Germany, Canada, Australia, USA and Italy...
Here's a link for anyone wanting to find a 'good' Montessori school - what to look for -
http://montessori.org.au/schools/choosing.htm
AMI does not accredit schools ...but individual Montessori associations in various countries do - and each one has it's own criteria and it's own way of doing this...in the USA they offer a certificate of 'recognition' and the criteria is here...
http://www.montessori-ami.org/ - look under "AMI/USA - Schools - Standards"
There are plenty of good programmes around - but always go and look yourself, look a few times, sit and observe, breathe in the emotional 'feeling' of the place...it should be clean, organised, peaceful as well as lots of activity, and the staff should be pleasant, available and not 'overbearing'...that said, I've met some Montessori teachers who are fabulous with children and hopeless with parents...