Is there a website that turns japanese sentences and turns the words into kanji?

Topic. For instance にほんご into 日本語

Thanks

If you've ever used an IME, you'll soon realise that automatic conversion is rife with sources of possible errors (there's a reason there's a word 変換ミス).

If the problem is that you're stuck on a computer that you can't install an IME on, (school, internet cafe, etc), there are web-based IMEs such as Ajax. As with all input systems it is up to the user to check that the conversion is correct.