Bitcoin is decentralised and anonymous so how can governments issue /enforce taxes on any transactions made with bitcoin?
Governments can set up laws and regulations and then set up "departments" to monitor and enforce.
Whether it is successful or not is another question. No doubt they will have to skill up and understand how digital currencies really work, maybe monitor a few people, monitor the flow of their btc's and audit them, create some "high profile" cases to be made examples of etc.
One can draw an analogy with cash.
Bitcoin is (...) anonymous
You won't go to the governmental office to say you have created a wallet. But they have huge control over your ISP probably and can check what are you send/receive to/from Internet...
The government wants you to convert back in the future, then pay taxes on capital gains and sales that follow.