My daughter came to England with us last year understanding a lot of English but Japanese was her first language. Within a year she now uses English predominately, thanks to a year spent in the local school's nursery. Like other EAL (English as an Additional Language) pupils I've met in my own workplace, her first reaction to a new language in nursery was one of shock and quietness. Children are listening, assessing and assimilating the new language in this first phase, before they start to communicate. I think my daughter spent 6 months at her first nursery doing so. The staff all thought she was very quiet...so NOT true!! At the nursery she started in January of this year, she was also thought of as being quiet but I think they are seeing her true colours now. Her English is also amazing, full of imagination, questions and excitement. She has had some wonderful and sensitive help from her present nursery; we have seen a huge change in her confidence and language since she has been there.
Here's a link to Dorset County Council's EYFS policy for bilingual children:
Welcoming Bilingual pupils into Primary Schools’ – Dorset County Council