I do not own Prince of Tennis. But you knew that, didn't you?
Time and Selection
When Ryoma first wakes up- in his bed, on the roof, under a tree- he can't remember Japanese. Often there will be someone standing over him (Momo, Eiji, Oishi, once even Fuji) and they will be speaking to him in varying tones of annoyance, waiting for an apology or an explanation or something.
Ryoma will have to pull his cap low over his eyes and wait for them to cease assaulting his ears with gibberish, because it's not like he could answer them- they wouldn't understand him any better then he could understand them.
It can take up to an hour for Ryoma to regain his language skills back. During that time he will do everything in his power to avoid conversation with others, and if he must communicate, he does so without words and prays he's interpreting their expressions correctly.
And he doesn't get it back all at once. That would be too easy.
It seems to come back word by word; first words that most American kids know- konnichiwa and sayonara. Then the first words he studied- numbers, animals, the other greetings…
It is only after he has recalled these inane words that the words necessary for conversation will come back; daijobu, and desu, andwatashi, and anata, and demo- not in order, of course, or even in grammatically complete sentences, just there and with only the barest of meanings.
After that he will finally start to recall what all of the suffixes are for and whatsensei means. Then Ryoma will remember how to fit it all together in understandable thoughts. Even after that takes a good twenty minutes before he regains his ability to think in Japanese thus allowing his fluency to return.
Ryoma has gone through whole classes completely unable to understand what the teacher was saying.
It's frustrating. It's aggravating. It makes Ryoma incredibly grateful that tennis- despite what some members of his team might think- does not actually require speech.
But it's not what frightens Ryoma the most.
Because sometimes- never as often as his loss of speech, but sometimes- sometimes Ryoma will wake up under a tree and not remember where he is. Someone will be shaking him, speaking to him, and Ryoma won't remember who they are.
What their names are. Who they are to him.
They are the first real friends he's ever had, and until he remembers they don't mean a damn thing to him.
He's looked it up. He couldn't find anything remotely like what ever was wrong with him. The closest he could find was culture shock or selective amnesia.
But culture shock fades with time and selective amnesia requires selection, and Ryoma can't figure out what's wrong with him.
It's been a year and a half, and when Ryoma first wakes up, he can't remember Japanese.