Thanks again, I've fixed it.

Thanks Daniil for your comment.

1. I've fixed kata's details.
2. What do you mean exactly? Do you suggest that January is a special case and we should hard-code "You are on track."?

As I am not native speaker, probably the decription could be much better. I am open to any suggestions.

The problem that I wanted to describe is about a possibility to check how well someone's challenge is progressing.
Let's create an example. I've decided to do 100 katas in 2018. It's beginning of March and I solved 30 katas. I would like to get information how well I am progressing. With this short program I can get information that I am ahead of schedule with 12. Program parameters in this case are: 100, 30, March.
I hope it somehow helps.

I've added random tests with use of your solution. Thanks for the comment.

Thanks!

I've just checked test cases and it looks like NPE could be thrown only if the input string is null. But all test cases are using correct month name.
So it looks to me to be an issue in your solution.

Deleting a language is not working - looks like X button next to language name is doing nothing.
How can I remove solution from incorrectly added language?

And another translation: for Java.