If the child is growing up in the UK then in my experience English will eventually become the child’s dominant language. This is because there will be more exposure to English from radio, tv, films, casual conversation, English speaking friends, nursery and eventually school. In school English will be the language the child learns in , the language in which they develop a wider vocabulary, learn to access cues in reading, use and understand the technical language associated with science and maths, the language in which they learn to write expressively, write persuasively, to write accurate accounts and make coherent arguments.
It will require a lot of determination from parents and family to support the other languages so that they become more than a simple language used for family discussion. Reading, writing, determined vocabulary building, using adult syntax, it takes effort and often outside support like additional classes. For many parents it becomes too much and other languages sadly fall by the wayside as the child matures.
From your point of view speak to your grandchild in the language you normally use, what really hampers language development is a child hearing poor language models, whether from a parent whose second language is minimal but who feels they should use that language because it is the dominant one in the culture, or a well meaning grandparent who can only use baby talk. A child needs good language role models to develop an understanding of how language works, and it doesn’t actually matter which language provides the model as long as one of them does.