How can I encourage my child to learn a mother tongue which we don't normally use at home!!

By watching TV! I learnt to understand Mandarin when I was young through watching Channel 8 serials with my late grandmother. I'd read the English subtitles and translate them into Malay for my illiterate grandmother. I learnt Swedish by watching TV too. I wouldn't say it's the best way but you can use it as a supplementary method. When you're watching TV and listening to the words while reading subtitles, your brain really needs to process things faster and I think that helped me grasp words and pronunciation faster. Here in Sweden, my friends would do the same -- let their toddlers watch shows in English since it is not their first language.