[Freak_NL]

For written Japanese, a lot (for the kanji). For spoken Japanese, it helps with the words that were borrowed from Chinese — and this is a lot, perhaps 50%. But pronunciation is, obviously, recognizable, but differing. For such words (those that consist of characters that are pronounced using the so called on-yomi reading), you're likely to pick on the sound conversion from Chinese to Japanese and vice versa, and at that point make educated guesses to the meaning of those words. That leaves pure Japanese readings of words of course (which includes almost all verbs excluding the nouns that are verbified by suffixing with an inflection of suru), but it helps in the basis.