Yesterday I attended the welcoming speech by the Vice Chancellor of UBD to the new intake of students. The first few minutes were entirely in Malay (as is appropriate when Malay is the official language of Brunei). He then switched to English for a few minutes (as is appropriate for a university that is mostly English-medium). But as he continued, he started increasingly switching back and forth, often within a sentence. This was clearly done to convey the informality and friendliness that he felt was suitable for a welcoming address. I wish I had taken notes; but I remember he started one clause with kalau ('if') but then finished it in English, and there were many, many such instances.
It is really interesting to see how such frequent switching between Malay and English is seen as the way to show informality, even on the occasion of a welcoming address to incoming students. My guess is that virtually all informal discourse in Brunei is characterised by this kind of switching and mixing.