(The) Japanese

Yes.

No, here it is not optional, T. And it might be, just might be because it isn't a plural form.

"Americans/Canadians/Germans make excellent cars" all sound natural.

I agree with JTT. It's an odd one, this.

Africans make fine athletes
Welshmen make great singers
Teak makes very durable furniture

but

The Japanese make excellent cars

but also...

The Welsh make great singers.

Here are they not seen as a single group? After I posted and read MA's reply, I started thinking it over.

I still think using 'the' is more natural and/or formal, and I'm not equating the two. But in casual speech, I think you could hear,

Japanese make nice cars.

My Dodge runs pretty good, so
The Americans make good cars, too.

If it came across that I thought 'the' is excluded for Americans, Buffalo, let me say now that I don't believe that.

Japanese are wonderful people. [could mean Japanese nationals and/or people of Japanese descent]

"The Japanese are a proud people"

Only the people of Japan???

"The Japanese I know ..."

['the' needed because of the specificity from the relative clause {(that) I know} could be solely a reference to ethnicity or it could include Japanese nationals]