Is it true that most japanese don't know how to read ROMAJI? and 2 other questions about japanese?

Question:

I read that in some of the many pages where i'm learning japanese.

Also i've read that japanese don't know how to pronounce the 'l', as opposed to the chinese that don't know how to pronouce the 'r'. Is this true?

Oh and... I've been learning that there are basically 3 different forms of speaking in japanese:
The polite one, for example: Watashi.
Then the common one amongst friends, for example: Boku.
And finally the one that is considered rude (or so i've read) for example: Ore.
My last question here is: Is it really rude to use that kind "rude" form of japanese? Will the japanese think of you as a disrespectful person? Some time ago i used to say: Ore wa; but after i read this i instantly changed to using: Boku wa. (bonus question: is the particle i wrote after boku written: "wa" or "ha"?

(ANOTHER BONUS QUESTION: Hoy do you say in japanese these 2 prases:
1. Are you japanese?
and 2. Welcome to my country.

Thanks a lot in advance.
I'm completely enthusiastic about learning japanese and i'll greatly appreciate any of your assistance here.

Answer:

I think you might be a little mixed up. Your example of the word "I" is both right and wrong.

The formal version of "watashi" is "watakushi". Both are gender neutral. However, "ore" and "boku" are both used primarily by men (and occasionally women trying to sound tough). "ore" does sound rougher and would not be used in business or polite society, but it is not exactly rude. So, if you are a guy and not in a business setting, you are probably fine saying "ore"

Now here is where your are right. Japanese does have three levels of speech but they are most often reflected in verbs and the addition of honorifics ("o" or "go")
For Example:

Casual: taberu (to eat)
Polite: tabemasu
Formal: meshiagaru (when someone else is the subject/object)/ itadaku (when you are the subject/object)

They all mean the same thing. With your friends and family, you will probably use the casual one. With your teachers, you would use the polite one. With your boss or someone you really need to impress, you would use the formal one. The formal language (called keigo) often uses a completely different verb all together. That is not something you need to worry about for now though. If you use the polite form, you will be plenty polite enough to get by.

Lastly, since no one answered this:
1) nihonjin desuka?
2) amerika e youkoso. (saying "watashi no kuni e youkoso" sounds weird)

*The particle は, is written with the hiragana for "ha" but pronounced "wa". The romaji is more or less up to you.