Knowledge hold by primates' brain does not follow an atomic structure,
but has a 'network' structure, determined by the links offered by the context in
which the 'elementary piece of information' has been first met and in which it has been lately recovered; as long as we are exposed to coherent language contexts
we will not mix languages, as our personal language knowledge network will grow with links to the same language; it has been suggested for helping this separation process with children living in multilingual families, that each member of the family will speak only one language, but eventually Nature seems to happily find its way
not mixing things;
of course also the inter-languages morpheme structural differences help in our categorization process.