I'd learn English just so you have access to more information in general. I'm thinking if learning mandarin just for expediency.
I was speaking to an old Chinese guy at work the other day, he told he his English was "sh*t" I told him his english wasn't that bad and asked how long he had been in Australia. I was assuming less than a year because he struggled to have a conversation in English and mainly relied on pointing and single words.
He told me he has been in Australia for nearly 30 years. Well he said "thirty" anyway.
I replied, "yeah your English really is sh*t." I want to learn mandarin so that I can tell them fluently in their language that it's rude to live in a country for 30 years without ever bothering to learn the language.