Once you've mastered the basic syntax and control constructs of a language, it probably isn't worth your time to try a dozen different string manipulation exercises.
Perhaps this is beyond the scope of what this site is aiming for, but, I would hope for some exercises that help me understand what each language is commonly used for.
Potential examples:
- Swift: Learn to display and manipulate a GUI
- Ruby: Learn to write a web application
- Python/Julia/R: Learn to fit some data to a model
- Javascript: Learn to use a web framework
- Go: Learn to handle some network connections, concurrently. (I see they have one exercise that emphasizes concurrency, which is a step in the right direction.)
- Perl/PHP/Bash: Learn to cry bitter, sorrowful tears.