Disclaimer: I own only this idea. Everything else belongs to Ohba-sensei and Obata-sensei.
So, what if L couldn't speak Japanese? They would need a translator, wouldn't they? Whatever is written in bold (after this A/N) is Japanese, just to let you know.
The task force walked in the apartment to find a man, about twenty years of age, with black hair and no shoes. 'I'm L.'
Soichiro Yagami looked at the other Japanese in the room, to see if any one of them understood English. They all shook their heads no. 'I'm sorry, but none of us can speak English.' He said, hoping L would understand.
He simply raised an eyebrow. 'Since you spoke Japanese, there is a 78 percent chance that you do not speak English. Unfortunately, Japanese and Czech are the only languages I do not speak. But me telling you this is absolutely useless, since you have no idea what I am saying.'
The task force again looked around at each other. 'We should find a translator.' Yagami said, running his fingers through his hair. 'Someone we can trust, who won't leak information. Who is defiantly not linked in any way to Kira. Hey, we could ask my son, Raito, to help us! He is very good at English! And he has no ties to Kira whatsoever, because he is my son!'
So, in a form of demented sign language, they were pretty sure they got across what they were trying to say. After they left, L turned to Watari. 'They are going to get the moon so they can finish the third level in a video game? Why would they do that?'
The next day they returned with the not-related-to-Kira translator, Raito. When they entered, L was sitting in a chair, eating a cake with pink icing. 'I see you returned. Did you find the moon and finish the third level yet? And who is with you?'
Raito smiled. 'I will be your translator today. It is nice to meet you, L.'
'Please, call me Ryuuzaki, for safety's sake.'
Raito relayed the information, keeping in mind the fact that he had a lot of power right now. 'He wishes you to call him Ryuuzaki, because he doesn't want the aliens to know where he is.'
Yagami gasped. 'Aliens? Is this man crazy? Raito, please don't tell him I said that. Make up something.'
Raito nodded. 'He said you must be crazy, and why are you so paranoid?'
L put down his now-empty plate and ignored the question. 'Please, if you all sit down. I will have Watari bring you drinks.'
'Ryuuzaki asked us to sit down, and that his servant can give us something to drink so he can give us truth serum.'
Yagami gasped. 'Truth serum? Doesn't he trust us? Also, please don't tell him what I said.'
'He said that you are very peculiar and is not sure if he wishes to do business with you.'
'Well, he doesn't have to. He can leave whenever he wants.'
'He said that you have a funny mustache.'
'Tell him he sits funny!'
'He will put up with your strangeness for now.' Raito's acting abilities were the only thing that kept him from smirking.
'That is good. Now, I have these special belts for all of you. If you push the button, it will send a message to Watari and we will know.'
'He has very fashionable belts that he wants you all to wear, because you will all look so good in them.'
'I believe Kira is a teen male, with a role model in some form of law, weather it be lawyer, police officer, or anything of this kind.' L told them.
'He thinks Kira is a middle-aged woman, with three children and a dog. Probably stays at home. Her husband is most likely a mail-man.'
A few weeks passed, in which the team amazingly stayed with L, but there was no advances on the Kira case. They had a very poor picture of L, just as he had a similarly bad picture of them. The only intelligent one seemed to be the translator.
At one session, L pointed to Raito. 'I am 32 percent sure that you are Kira!'
Raito translated. 'He is 32 percent sure that he likes me, and wants to date me because I am so sexually appealing.'
'Gasp! He can't hit on my son! Tell him that he is a pervert!'
'My father disagrees with you. He wants to know your true name, or else this investigation will be over.'
'I will not tell him my name. You all may leave.'
'He doesn't like being called a pervert. He told you all to leave.'
So the task force left. Raito stayed. L stood up and put his arms around Raito. 'Finally, I thought they would never leave.' He said, right before leading Raito to his bedroom and giving him a prolonged kiss. 'My little translator.'