[KyleGoetz]

Originally Posted by NightBird

So the real hard point is Kanji and the writing part of the language...
But about speaking, well I agree this is not hard...

Growing your vocabulary is hard (basically the only word you already know is クロワソン (croissant)), and kanji makes it easier.

In my country, some schools do courses in the afternoon (average time in course: 4 hours) of English, Spanish and ...
Is this kind of courses in Japanese will be enough if I take like 2-3 hours/day, 4-5 day/week?

Enough for what? You won't be fluent, but if you work very hard, you will be able to carry on a few conversations and read some simple written pieces in 3-4 years. According to the US foreign service (a department of the government that trains people in foreign languages, among other things), Japanese takes at least three times as many hours of study as French/Spanish/etc. to learn for an English speaker. It's probably pretty similar for a French speaker.

And If I understood, the "Kanji" is the Chinese/Japanese "alphabet-like" and Katakana and Hiragana are used especially for syllabes?

You got the katakana/hiragana right, but there is no alphabet. Think of kanji more as a combination of rebus (Rebus - Wikipedia, the free encyclopedia ) and prefixes/suffices like "anti-" "pseudo-" "-ment" "-tion" and then full words, too. For example, 機械 means "machine." 化 means "-ize." 機械化 means "mechanize."

Good luck.