"Mañana va a llover."
Translation:Tomorrow it is going to rain.
For what it's worth, I've heard some Mexican songs where those infinitive verb endings like -er get pronounced as -esh. Really confused me at first, especially when the same singer doesn't do it in other songs! I guess it's an accent thing
Ll as a j sound is definitely an accent. It can be pronounced like j or y or sh (and variations in between) - I don't know how consistent duo is with its accent, but it's good to be aware in case you hear it in real life
The -esh sounds at the end of llover (and potentially all infinitive verbs and many other words) is a devoiced trilled 'r'. Since it's the end of speech, there is no use in keeping the voicing up, and since the trilled 'r' is one of those sounds that have a wide range of places where you can form it (from the back of the teeth until behind the alveolar ridge), it can sound similar to an equally wide range of sounds if it's devoiced, mostly 'th', 's', or 'sh' sounds.
I understand what you are getting at here, but you have confused the issue a little. You are right that se va a llover is a grammatically incorrect, but common way to say "it is going to rain", but the se isn't needed because, unlike English, the verb strongly indicates the subject.
But the problem with that question is less that the sentence has an extra pronoun, and more that irse (which is the only potential reflexive in that sentence) means "to go" in a literal "moves oneself" or "to proceed" sense (se va bien = It is going along okay). So, since there is already a specified noun, it becomes a reflexive on mañana indicating movement.
In short Mañana se va a llover. reads something close to "tomorrow is moving to rain".