My thoughts on this are only that a solid base in the "home" language, be it English, French or Greek, will not be affected by going to a monolingual school. The big caveat is that the home language is consistent and well presented (no dees, does, ain'ts and dem if you get my drift.
Also encouragement to follow the home language on television and movies as well, oh my God how important!!, as READING. Just a few paragraphs and then pages and then books....This is the foundation for every language.
And if the child is very young I would not sweat it all that much. Kids are like language sponges. There are some of the bilingual schools that produce really nice English speakers....like Santiago Christian and St. David....Carol Morgan of course and I imagine that there are other good ones in Sto Dgo.
Try it any way you want, and see how it works out. In today's society, I do not think that language is as defining as it used to be.
Cordially,
HB