Create Create sl.json

[miropuhek]

Slovenian translations.

[miropuhek]

While translating into Slovene, I noticed what I believe are two typos in the .en.json file. Could you please review the following lines:

• Line 311: It seems to be identical to line 307, but perhaps it should read, "Will be played when an exercise is closed."
• Line 588: Is the period here intentional? ". Exercise for @stagelabel""

[otacke]

@miropuhek Thanks for being so observant!
The first one was in fact a copy&paste error where I forgot to replace "opened" with "closed".
The second one is in fact correct. It will be concatenated to the stage title that is fetched from the exercise. But I can see how this can be confusing. I'll change this.