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Overture and Wordtracker are a good place to start. Just enter in the translastions of the keywords you want to target and bingo.
The only problem is that some of the results are very poor at the moment. It appears that they are not collecting enough data on various languages.
Check out some keyword lists and message boards, for example, and you will see that most people type without the accents unless it is needed formally or there is a clear distinction where certain words need the accents. In most cases, however, people will know the distinction in relation to the phrase or sentence that is being used.
Now, that isn't to say that people will not type the accents, therefore it should be on the page translations, but you should also target keywords without it as well.