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Some exceptions are abbreviation or well recognized names especially computer or internet related. i.e. Windows (name of OS), Java (name of programing language), USB, DVD.
There is no Japanese version of wordtracker, but you can use Japanese overture's keyword tool.
Need Advice on Japanese SE's [webmasterworld.com]
Best English-language SE to reach asia market? [webmasterworld.com]
It is simply a fact of life. Japanese language pages have a lower PR than a similar English language page would have.How long does it take for DMOZ to impact on G?
Japanese pages have lower Google PR? [webmasterworld.com]
So having some English content can help to get links (and some PageRank) from English pages.
Those are just some general ideas, but I think you get the idea. Like the others have said, your Japanese language traffic should be a lot higher. Do take a look through those threads that takagi noted as there is some good discussion in there.
have done some very simple research using o/ture and adwords on frequency of searches in japan.
using keyword "new zealand" in english and japanese as an example
98% of searches in Japanese on O/ture japan
2% of searches in English on O/ture Japan
i would imagine that this would vary with the technicality of the search term: i.e. more technical the term (e.g. scientific or engineering jargon) the higher frequency of English due to the penetration of english tems in technical fields. also be aware that transliterations into japanese katakana can often have multiple spellings (try www.alc.co.jp for English - Japanese dictionary to chekc for spelling variations)
so while you may not get any results from japan on an english search, like other pples comments it cannot hurt to optimise for the english as well.
have done some very simple research using o/ture and adwords on frequency of searches in japan.using keyword "new zealand" in english and japanese as an example
98% of searches in Japanese on O/ture japan
2% of searches in English on O/ture Japan
i wonder if there is any evidence of the reverse? o/ture translating in the other direction? if they can do it in one direction, they sure can do it the opposite way round.
www.language.domain rather than www.domain/language?Actually neither of those from my experience. If possible it is preferable to get a .jp domain name for the Japanese language site. I have seen better traffic from moving from a www.domain/language to domain.jp. I think you'll see this is generally the consensus with other languages as well (e.g., Europe Forum [webmasterworld.com]).