What do you think of the skill level of Japanese software engineers?

I think it probably matches the skill level of French chemical engineers.

Some are excellent.

Some are average.

Some are poor.

Just like software engineers everywhere else in the world.

Some great software used around the world originated in Japan, for example the Ruby language, thanks to Matz, which is used to build all Ruby-on-Rails webapps. Twitter began as a RoR webapp. Bloomberg, AirBnB, SoundCloud, Heroku, Hulu, Zendesk, Github, slideshare are a few other famous RoR websites. Ruby is a joy in which to program.

What a strange question.  How would 99% of us be in any position to judge?

Hahahahhahahhahaahahahhahahaa. Sums it up nicely.

Twitter began as a RoR webapp. Bloomberg, AirBnB, SoundCloud, Heroku, Hulu, Zendesk, Github, slideshare

Sure, they started out that way because it was popular at the time, but since then RoR has fallen out of favour and most of those sites are different now due to performance and scale reasons. Using the Wappalyzer plugin for your browser you can see pieces of the stack used in a website.

Github is still Rails --

Heroku has Rails, Erlang, Cowboy, lots of JavaScript --

Airbnb a mix of Rails, React, webpack --

Twitter is React/webpack --

Bloomberg, Soundcloud predominately JavaScript webpack --

Hulu with JavaScript, jQuery, Node, React --

Zendesk, jQuery --

SlideShare, JavaScript handlebars, jQuery, ZURB Foundation --

I also used RoR a while ago, but moved on to React and JS libraries. It's called Atwood's Law: any application that can be written in JavaScript, will eventually be written in JavaScript