My older son went there in the Spanish program and I know two people who have kids there now - Spanish and Japanese programs, and are happy with it.
The school was going through a lot of turmoil back then - 4 years ago - and I know they have a new school head since then. However, the parent community and board is very good. Ask about discounts for tuition - I'm not sure what they do now, but we had a needs-based discount when we were there and I believe they still do that, especially for multiple kids (you don't need to be poor to get a discount, just unrich :-)
I think it is a good school. My son was speaking pretty fluently after just 18 months in the program (he lost it all the next year when we left - make sure you keep up the language!) They aren't lying when they tell you that the kids from their programs are better-prepared for school and that they pick up the language. But like anything else, it's really a fit - your family, your child, the teacher - especially the teacher, since it's a 3-year cycle.
You might want to visit some of the other small montessoris in oak park, too. Especially if your interest is Spanish, there are several Intercultural alumni that are running small schools now. You can email me and I can try to find contact info. My son's old teacher has a lovely little school out of her home (AMS certified) and she is awesome. It's also more flexible - you can pick a part-time option if you want, I believe.