Changing language does not correctly change the default comment

[raivatshah]

Steps to reproduce:

1. Go to Source Academy Playground
2. Default language should be JS and there should be a comment "// Type your program in here!". Change language from JS to Python
3. The comment stays the same, which causes a syntax issue.

Expected behaviour upon language change should be to automatically change the default comment as well. I believe the same issue will also happen if the language is changed from JS to Scheme.

[RichDom2185]

Part of larger #2463 efforts in general. The placeholder comment should be part of this language config object. I'll take note of this.

[raivatshah]

Thanks @RichDom2185!