I've tried to formulate the question so as to exclude non-Christian/Catholic ideas of prayer as well as "tongues" which might be a result of a more charismatic style. In addition when one moves to the part of prayer which has no need of words (contemplatio or insert your favourite spiritual writer's term in here) the question is moot.
For reference, my L1 is English and L2/3/something is Dutch.
For the Divine Office or Mass, depending on externals (what language it's scheduled in, which book is nearest, etc) I can use either and am equally comfortable in both.
For the Rosary I often find myself switching mid-decade between any of my most familiar three - English, Dutch or Latin (I'm conversational in other languages but not to the extent I could pray in them).
For "mental prayer" - that is, silent prayer which still makes use of unspoken words - I find myself equally able to use Dutch or English.
My reason for asking is that many of my friends are at least competent in both English and Dutch, and would be able to certainly follow Mass or Vespers (etc) in either language. Some would also be able to pray the Rosary in either (my penchant for randomly switching seems to be a quirk of mine). But it seems that barely any would be able to do the third - hence my curiosity.