tl;dr:

• granola (sugary crispy baked oats etc) ≠ muesli (soaked soft raw oats etc)
• they both started out as trade names and became more generic terms
• they're both ultimately breakfast products that may be viewed as analogous despite their manifest differences

Muesli is Swiss, and both that and Granola were originally more specific trade names that have come to refer to more general products. Granola and Muesli are very different from each other in what they are, but certainly fall into the same category of thing (oat-based breakfast product).

"Originally known in Swiss German as Birchermüesli or simply Müesli, the word is an Alemannic diminutive of Mues which means "puree" or "mash-up.""

("Bircher" comes simply from the name of Swiss physician Maximilian Bircher-Benner, who introduced it for patients in his hospital)

https://en.wikipedia.org/wiki/Muesli#Etymology

Granola is similar, though definitely not the same thing.

"The names Granula and Granola were registered trademarks in the late 19th century United States for foods consisting of whole grain products crumbled and then baked until crisp; in contrast with the sort of contemporary (about 1900) invention, muesli, which is traditionally not baked nor sweetened."

https://en.wikipedia.org/wiki/Granola#History

All that said, while granola (sugary crispy baked oats etc) ≠ muesli (soaked soft raw oats etc), the idea is close enough that I wouldn't be terribly opposed to its addition. It's a cultural thing, like in the Italian course where there's problems with the translation of deep pasta plates, of the kind not really used much outside of certain parts of Europe.

Wonder if "cereal" is accepted (i did not try). Muesli is a type of cereal, no?

Not really, but it's very common in Ukraine.

But... not so common in some countries :')

If I were the boss, I would remove all these local words like сало, мюслі, маршрутка etc. and put some vocab foreigners can relate to, vocab that is not so confusing... But well :/

No, though it's a kind of cereal product (see my other post about the name and meaning).

Nobody said Ukrainians invented it :)