TL;DR: Bitcoin has been declared a legal form of payment which is not the same as an official currency.
I think that it may not be clear what a "legal form of payment" means and how this differs from the official currency of a country.
"Currency" usually refers to a form of money issued by a government, which makes "cryptocurrency" a misnomer to some degree.
Currencies generally have a high level of protection and status in their country, for example the nation requires the taxes to be paid in terms of the currency and usually requires its population to accept the currency as a means of payment. However, first and foremost the currency defines the unit of account that is being predominantly used in the country.
Yet, nations recognize a number of different ways to settle obligations explicitly which are sometimes called "legal forms of payment". These may be different forms of transferring the nation's currency, but can also be other forms of money. For example, if you are using a form of money that the nation has not recognized as legal form of payment, in the case of legal dispute it may first need to be established whether the payment was valid in the first place, and if the payment is not recognized as such, the value may be lost.
Some examples of legal forms of payment are cash, money orders, checks, and wire transfers, and in most countries also digital payment networks such as PayPal. Although uncommon, payments settled in foreign currency will also be recognized. In some regions around the world there are local currencies that aim to encourage commerce in their region. Often these regional currencies are tied to the national currency, but given out at a discount, e.g. the Reka-checks can be bought by employees in Switzerland for 3-20% less than their face value but only used to pay for tourism services in Switzerland.
To my understanding, what happened in Japan a few days ago was the recognition of Bitcoin as a legal form of payment. This does not give Bitcoin a special status, except that value transfers performed with Bitcoin are now explicitly recognized as payments in Japan. Similar things have happened in the past, e.g. when Germany declared that Bitcoin would be treated alike to a complementary currency, or (IIRC) Belgium declared that it would treated like foreign currency.
An overview of the current legal status of Bitcoin in the world can be found on Coin Dance.
Note: I'm neither a lawyer, nor an economist.