I'm writing a compliant letter to a Japanese company. Help please?

If they produce products for foreign markets it's very likely they have someone who can read and speak English, so you should not try to write it in Japanese or use a translator unless you are fluent in Japanese. Also, don't worry about excessive politeness because a letter of complaint (I think you meant "complaint", even though "compliant" with the i and a switched is a real word with a different meaning) is not supposed to be polite in any language. Just write it professionally and respectfully like you would any English letter, but don't be afraid to state your points as they will want to know if there are problems with there products so they can improve them.