Smart Contract token Ones tokens are issued by smart contracts on a existing blockchain, for example Bitcoin and distributed to the community, who and in wich currency the transaction fees will be realized ?
This is highly implementation specific. 

For common token platforms, namely Bitcoin and Ethereum, fees for transactions must be paid in Bitcoin and ether respectively.

Your contract or token mechanisms can impose a further fee in your token, or another token, but the platform itself requires the transaction cost to be paid by the native currency.

There are various proposals (1, 2), mainly for ethereum, that would allow tokens to be used to pay base transaction fees as well. However, these will require a hardfork, and are not currently usable.