What mistakes are Bob and Sally making?

I would call it "identity-wide generalization". They both suppose from something that is true of a particular situation (that is, learning a specific language or completing some projects) something more general at identity level (I can learn any language) that simply isn't true.

What would you change, so turn those mistakes into successes?

I don't know if anything can be changed to fix the mistakes. Take the first example: if Bob had the motivation to study French only because he felt the pressure of going to live in France, it's not said that there could be anything he can do to learn German, if he stays there.

Is there something in your life, that has failed in a similar manner?

I'll skip this since there's nothing that currently is failing for this reason.

To what other domains does this generalize?

I guess everything that touches upon betting on self-identity: "I had a lot of fun swimming at the sea, surely I'll be good at my local swimming contest", "Learning C# was really easy, I bet I can conquer F# even faster", etc.