Why is "i" not correct here?
That's just not the preposition Swedish uses. You always write på a language.
But in most cases "i" means "in" right?
Yep, far more often than not.
Afaik that is kinda like saying you write inside swedish
That'd be better for inuti. Using i simply doesn't make a lot of sense here.
I thought "om" was "if". Is there any setting where "om" becomes "in"?
Yes regarding time, om means "in". Example: Om en stund > "In a while".
Ingen fara! Glad att jag kunde hjälpa dig! (No problem! Glad I could help you!) :)
When we use pä ?
You mean på - pä is not a word. :)
Usually, it means "on", but as is most often the case with prepositions, it can have a very wide range of meanings not always directly translating into the corresponding English preposition.
Is "Svenska" a proper noun?
No, it's not.