Actually, in your case, what you want is a slightly different sentence:
I'm really looking forward to when my English is good enough that I can talk clearly to my foreign friends.
I'm really looking forward to when I'm going to be good at English, so my foreign friends can understand me well.
or, my preference would be:
I'm looking forward to getting better at English, so I can talk to my foreign friends more easily.
The future perfect tense (i.e. "will have become") is used for things in the future that will have been completed, but it does not matter when they were completed. For example:
- Three years from now, I will have lived here for 10 years.
- By the time you graduate from college, I probably will have retired.
The reason why you don't need the future perfect in your sentence (or any future tense) may become clearer with these examples:
- We say "when I grow up, I want to be a doctor" as opposed to "when I will have grown up, I want to be a doctor"
- Similarly, we say "Call me when you get to the bar" as opposed to "Call me when you will have gotten to the bar".
The expression "looking forward to" usually expects an "-ing" form, as in "I'm looking forward to seeing you again." You can add words after "looking forward to" and avoid the 'ing, as in:
- I'm looking forward to the day I see you again, or
- I'm looking forward to (the day) when we are together again.
So you can say "I'm looking forward to when I'm good at English", or use an -ing form and say "I'm looking forward to becoming good at English".
Finally, "with great clarity" means something a bit different - it's not so much about speaking clearly, but about being understandable:
- Our teacher knows how to explain even the most difficult concepts with clarity, so everyone in the class understands them.
- The user manual for this laptop is written with such clarity, that even a non-technical person can learn how to use it quickly.