not sure about Malaysia but used a lot in China, even those pisang goreng kind of stalls can accept wechat payment in China, normally the procedure is :
> scan the seller barcode.
> key in payment amount.
> select bank and key in wechat payment password (need to set up and verify first for first use.)
> show seller ur phone to prove you pay correct amount.
> grab ur item.

competition is very fierce between alipay/wechat pay in China so a lot of chance these payment methods get 5~10% discount compare to cash.

not sure if wechat accept non-China bank or is there any hidden charge, if accept and no hidden charge, its worth to try it as cash alternative.