So, the rule of the day was monolingualism : one (monos) language (lingua).
There are holes in the theory, because there are scores of people who speak English and a second or third language and speak all of them well. And then there are those who can only speak English, and, well....they certainly could speak it better!
So, I suspect, it isn't limiting oneself to one language that ensures that you speak it well. Instead, it is being schooled well in any language that ensures that you speak it well.
The Carolinians of Saipan have always impressed me because the majority of them have to learn how to speak at least three languages. Firstly, their mother tongue, then Chamorro and then finally English. The older ones born in the 1920s also learned a little Japanese before WW2, and then, after the war, a little English.
Mr. Lino Olopai, a well-known Carolinian historian and cultural expert, is one such polyglot. Recently, he spoke on Guam at a conference on Chamorro herbal medicine. He spoke in beautiful and perfect Chamorro about the need to preserve this folk medicine tradition and to unite as a people to work together towards this preservation. Unfortunately, I was able to tape only a tiny fraction of his short speech.
The clip is so short that I couldn't fit all the transcription notes onto the clip.