When on holiday/ dDuring her holidays she'd like to taste/try different kinds of fish in the seaside restaurants.

This is the most natural form for the sentence. You could say "when on holiday" but it sounds a bit... habitual, if you know what I mean? Like you might say "When on holiday, she likes to sunbathe" - that's her habit, not a specific plan for a specific holiday. If you add a verb - "When she goes on holiday she'd like to" - it sounds much better.

"Taste" is fine but "try" seems much more natural, taste here sounds a bit more tentative, limited. If I take you to a restaurant and I think their burgers are fantastic and you should order one for yourself - "You should try the burger", probably not taste. If I've ordered something for myself and you want me to give you a tiny little bit so you can find out what it's like - try would work there too, but also "Could I taste a bit of that?"

When you come to the Baltic Sea/ Baltic coast , you should try fishes like trout, cod, plaice or halibut.

I think with "come to", coast is more natural. Coming to the sea strikes me as slightly odd. Not sure if it's wrong, though.

Fish - this one's tricky. You have the right idea earlier - when it's meat, the plural is "fish" - but here I'm not sure. If you were talking about the *actual fish*, like buying living animals for an aquarium, then yes. Buy some fishes, like a trout, some cod. But here I think it's still meat. When you're trying trout, you're getting cooked meat delivered on a plate, not a creature.

I'm tempted to say "try some of the local fish, like trout". But I think "try fish" works.

Fish is sold by the weight and served with chips and a salad set (usually a combination of coleslaw, red cabbage salad and carrot salad).

Is it correct to say: "I have 4 fruits on my plate;: a peach, a kiwi, a mango and a fig"?