I think I need to reword what I said because I never said that a foreigner couldn't be a J-Pop artist. The point I was making was that learning the language will make the whole process easier. Singing in a language and not understanding what you are singing isn't a bad thing. As a music major getting my Bachelor's in Music Education I have sung in English, French, Italian, German, Japanese, Latin, Spanish, Greek, African tribal languages (can't remember which ones), Hebrew, Korean, Russian, and a couple of other languages whose names I can't remember. The point is that I don't speak any of those languages (except English and I'm currently learning Japanese) and I can still bring emotion into the pieces because I can listen to the accompaniment and read translations to get a general understanding of what I'm singing. All singers with experience and proper vocal training bring emotion into pieces they don't understand.
The point I was making was that doing the above is fine, but if you plan on living in a country, especially if you want to sing in that countries native language, it is best to learn the language. Since I plan on moving to Japan and teaching music there, I am currently learning the language, history, culture, and traditional music of Japan so that I have a firm footing when I get to Japan to begin my career.
Learning the language isn't necessary, but it gives you a huge leg up, especially in music. It is impressive to hear a foreigner sing in your language, but it is even more impressive if that performer can also communicate with you in your native tongue when off the stage.
If I planned on going to France to sing french opera for the rest of my life I would make sure that I was fluent in French. It would be the same no matter which country I decided to live in. I believe that a foreigner who wants to become an immigrant needs to know the language. It makes every aspect of daily life easier, no matter what profession you go into.
As for what you said about it is only one's talent for singing and marketability of style and vocal quality that is important. It doesn't seem to me that the pop music business doesn't follow that philosophy at all. That only proves how pathetic the music business has become. The "marketability" issue is what makes it pathetic. 50 or 60 years ago what was important was your voice and nothing else. Now it really doesn't matter what your voice sounds like as long as you are attractive and don't mind flaunting your body. I'm not sure if that's how it is in Japan because I have heard some pretty horrible "J-Pop" singers, but I've also heard far more really good pop singers in Japan than anywhere else. It seems that in the Canada, North America, Central America, South America, and Europe, what I said is true because I hear a lot of pop singers with horrible voices, but attractive bodies.