The TTS pronounces it "internatt-sjonell", and that is correct (at least from what I've heard on Swedish TV). There is both a "t" and a "sj" sound in there. I think it's an exception. And the "sj" part of it can vary, it can sound like "hw" or "sh", depending on the speaker's dialect.
I agree. I came here because I'm a native English speaker and I've never heard a person referred to as 'international'. I think the meaning this sentence is going for is 'cosmopolitan' but it could just as easily mean they're a mix of different nationalities. However, both sound weird.
There's no difference in meaning between mycket and väldigt when used before an adjective like here.
You can also use mycket to mean a lot, like in mycket pengar = a lot of money and you can't do that with väldigt which only means 'very'.
jätte- is indeed very informal. It would sound a bit odd to combine it with internationell here.
I just think that the adjective "international" simply does not work referred to a person. It sounds really cacophonic both in English and in my native language. I mean, i would kinda understand what you are trying to say but it's something i wouldn't expect to hear at all. Anyway, that's just me