In Malaysia, when I deal with government departments, I use Malay and sometimes English.
In business, I use mostly English. If the customer is an ethnic Chinese, I might use English, Mandarin or a Chinese dialect. If the customer is an ethnic Malay, I might use Malay or English. If the customer is an ethnic Indian, I might use English or Malay.
Among family members, I use various Chinese dialects and English.
When dealing with foreign workers, I use Malay.