Now I think I see your situation better. First of all, apologies for using "salary" above. Salary would refer to income earned as an employee. However, you are acting as a freelancer or independent contractor.
Just to confirm, are you living in Japan? If so, 20 percent sounds a bit high. Typically 10 percent is withheld at the source for professional fees paid to a resident of Japan (regardless of nationality). The 20 percent deduction is withheld when it is to someone without residence in Japan (living overseas, etc.). So you definitely should see about getting that back. It sounds like the payer withheld too much.
As for the fees earned from the website translations, these would be handled similarly to the past translations you had. The payer would withhold 10 percent and you would receive the rest. If you are receiving income, fees, etc. from more than one source, you will need to report this to your local tax ward in Japan via the kakutei shinkoku form.
As for reporting your earnings from Japan to the US, the IRS requires that US citizens report foreign-earned income, so you will need to fill out your 1040, etc. annually. There is an exclusion for earnings up to ~90,000 USD, so as long as you are making less than that, you will not have to pay more to the US. If you make more than the exclusion amount, you'll need to pay taxes to the US on the excess amount.