I know of a few couples who use an intermediary language, such as English, and to date perhaps about half of them have separated which is the same ratio as in the general population. I know of even more who started with one not speaking the other's language but where that was the language spoken where they made their home but eventually the "foreigner" became reasonably capable in the adopted language - again, not dissimilar end outcomes regarding separation. Not all children (perhaps a bit more than half) in mixed language families ended up learning the "foreign" language, some didn't because they lacked the interest but in most cases it was because they were not exposed to the "foreign" language because that parent used either the intermediary or the "native" tongue when talking to the children. In the former case of the apparent failure to learn the "foreign" language the children still sometimes ended up understanding pretty much everything they hear spoken in that language.