Of course you can use dictionaries! Just make sure to give them credit if it is required so under license or you just wanna do so. Also I think it would be awesome to have some of more unusual symbols included too as they are really handy if you are into coding but thats my opinion. This is your game though so its really up to you to decide what to use/not to use. I am much more interested in making learning typing more exciting and accessible to everyone. The other goal of this jam of course is to encourage making games because its awesome and fun.