Learn English – Is ‘color’ countable

The word toast in the sense of "toasted bread" is an English coinage from the early 15th century and originally referred to bread that was added to wine or ale for flavour (and possibly to soak up the dregs). In that context, a mass noun made more sense than a countable one, since toast didn't come in slices. It was only in the 17th century that toast started being eaten on its own with a spread.

The word toast in other languages meanwhile (such as Italian) is a modern borrowing, and so is more likely to be countable.

Update: a bit more investigation shows that the situation is more complicated than I suggested above. Early examples in fact sometimes use toast as a countable noun, despite referring to a piece of dipped toast: for example "Go fetch me a quart of sack, put a toast in it" from The Merry Wives of Windsor (1602). I'm not sure when it became exclusively uncountable.

Your search for a rule is admirable, but alas! doomed to failure.

• The plural of fish is fish. Unless you're differentiating between species:

The smaller fishes are more affected by ocean warming than the larger.

Or if you are a mafioso, in which case you say

Vinnie sleeps with the fishes.

Of if you're a theologian discussing the miracle of

the loaves and fishes.

Different species form plurals in unpredictable ways. Both tuna and salmon are their own plurals. Species that end in -ing, like the ling form their plurals by adding a final s, except for grayling and herring. You just have to look it up here. However, if the fish name is the name of a special at your restaurant, you might hear a waitress call out

I need two salmons and three tunas!

She means two orders of the salmon dish and three of the tuna dish.

No matter how many you have in a bowl, you only have fish, never fishes.

• Cakes is the plural of cake. It never means pieces of cake:

  Some cakes have frosting; others have icing.

• The plural of fruit is fruits, but only when you're talking about different varieties:

  Some fruits -- bananas, apples, kiwis -- are good for you. The rest are not. You always eat some fruit.

• Drinks are the typical nonountable nouns, except when you're talking about varieties or individual servings:

Whiskeys are either blended or single-malt.
Give me two whiskeys, two scotches, two beers.

Waters has an additional plural as the naturally occurring water in a location, so during your vacation, you

take the waters at the spa at the hot springs

• Cheese follows the variety rule. If you have three cheeses on your cheese plate, then you have three different types of cheese, even when you have six pieces of cheese total. Same with milk:

I make three different nut milks in my blender -- walnut, hazelnut, and almond.

The plural of beef is beeves, but it's only used to describe individual animals, meat-on-the-hoof, so to speak.