Since it's written in English, let's switch back to English. Would be weird for me to append comments in Japanese to excerpts in English.
First things first, you did a great job. I'm sure you've noticed over the years that I have a problem with verbosity, and you managed to write succinctly. I also like some of the points you raised, like about people who say they have no time and how you compared mnemonics to constellations.
I noticed some minor grammatical/spelling mistakes, and I don't want to sound like a grammar police, but as you'll eventually get to the area of grammatical constructs like sentence structures, you'll have to be extra careful in laying out the differences between English and Japanese sentences as well as providing phrase-to-phrase or even word-to-word translations of example sentences.
For example, you wrote "contents" twice in "adult contents" and "side contents" where it should be "content". This isn't just a mistake of plural form where it should be singular—"contents" is what's contained in a container; "content" is material, substance, basically 'stuff', especially with regard to creative works or publications.
Next is your use of the phrase "code system". I can picture what you had in mind when you wrote that, but—at the risk of sounding like an elitist—I'm not sure if those not versed in linguistics would be familiar with the phrase. It'd be like if, in the sentence preceding the one containing that phrase, you used such terms as "Germanic languages" and "Romance languages" instead.
Additionally, the goal to "understand all [Japanese] words by using a dictionary" is perhaps overselling it. How about something more achievable, like "understanding the gist of a sentence even if you can't find all the words you don't understand in a dictionary"? Realistically speaking, beginners will have problems reading at least some kanji or catching what a character is saying, rendering them unable to look up the words. (cont)
