Does that mean I can refer to adjective-denoted nationalities without "the,"

No.

This is journalese and is usually limited to headlines. It also requires a certain form of context.

1. Conversation

Hello John.

Hello Mary. Can you tell me a fact about France.

French love good food. (incorrect)

The French love good food (correct)

2. News story

French love good food says Macron!

When being interviewed last night, president Macron was asked what he liked most about being French. "We the people of France, love good food!" He proclaimed.

Adjectives cannot be grammatical subjects

No, you cannot do that. The reason is somewhat trickier than you might imagine.

So to start with, of course you can say this:

• Spanish is an easy language for a Frenchman to learn.

Because it’s here the name of a language, Spanish takes a singular verb.

But you cannot say:

• ❌ Spanish are increasingly restless. [UNGRAMMATICAL]

That’s illegal. Since adjectives in English lack number, they cannot be the subject of a clause.

Instead you must use a plural noun and say:

• Spaniards are increasingly restless.

By using a plural noun, this of course takes a plural verb.

However, when you write:

• The Spanish are increasingly restless.

You do not have an adjective as the subject. And yet the word Spanish there is not a noun, either.

And this is where it gets a bit tricky.

What you do have is a clause that uses a fused-head modifier as its subject. The fusion is that the actual head of the noun phrase, some sort of noun, has fused into the modifier, but it is still a noun phrase nonetheless and so can be the subject of a clause. What it has fused with is some plural noun like people, which is why it still takes plural concord.

This is the same thing that happens with fused-head constructions like these:

• The tired are with us still.
• The poor are with us still.
• The huddled are with us still.
• The wretched are with us still.

None of those sentences has any nouns in it. You can apply intensifiers to adjectives but not to nouns, and those can all take intensifiers without a hitch:

• The most tired are with us still.
• The very poor are with us still.
• The rather huddled are with us still.
• The really wretched are with us still.

If those had been nouns, you could not have done that. Since you can do that, those are not nouns. They are adjectives forming part of fused-head constructions. The original nouns are fused and gone.

If you want to use just an adjective and have that be an entire noun phrase, you need to have a determiner precede it to signal that the head noun has been fused.

There are other types of fused heads than adjectives, but we can save those for some other day.