The PC98 keyboard has three important keys for Japanese input: the kana key, NFER key, and XFER key.
Basically all PC98 emulators map these keys to the hiragana/katana key, muhenkan, and henkan keys on a modern Japanese keyboard.
If you don't have a Japanese keyboard, maybe it would easiest to remap keys with a Autohotkey script. That would work with every emulator without any modifications.
Download Autohotkey, then copy and save the following as a text file. Name it PC98keys.ahk for example.
Code:
; PC98 kana key to F12
F12::SC070
; PC98 NFER key to Home
Home::SC07B
; PC98 XFER key to End
End::SC079
Then just load the ahk script before using an emulator.
About typing Japanese:
On the PC98, you can input katakana by hitting the kana key, but that's it. If you want to type hiragana or kanji, you need to load a Japanese front-end processoer (FEP).
In MS-DOS, this is accomplished by adding these device drivers to config.sys:
Code:
DEVICE=A:\DOS\KKCFUNC.SYS
DEVICE=A:\DOS\NECAIK1.DRV
DEVICE=A:\DOS\NECAIK2.DRV NECAI.SYS
Once the Japanese FEP drivers are loaded, you can switch to Japanese input by pressing Ctrl+XFER.
But this is not normally used in games. Games either require typing in katakana by pressing the Kana key, or they come with a built-in FEP of their own.
For Idaten Ikase Otoko, you just type in katakana. But I think the password is different from the PC88 version.
I haven't checked Delfoi, but if you upload a save game or save state, I could take a look.