You could re-use the code for the hadokens to do the umlauts, also, something nobody ever did
I did manage this extraordinary feat before
However, it's not worth it, because:
a. trying to repurpose dakutens for accents yields ugly results
b1. the dakuten by itself often takes up the space of one character in the case of games relying on 8x8 fonts. Meaning で/ä would be 2 characters. Early Final Fantasy games (MSX, NES FF1) use DTE to make those a single character, but that's not true in general.
b2. alternatively, the dakuten data is stored elsewhere in a hard to figure out format, making text edition very cumbersome
And, while this pseudo-translation practice by itself is despicable, I find the review a bit overly dramatic
I mean, the project is a relic of an era where fan-translation was at its baby steps, and the author even admitted to making stuff up (much like SD Snatcher, a very similar other early translation project). And even Square/SCEA is no stranger to making stuff up in translations to make it feel more edgy/faithful (F-bombs and bleeps in FF7's English script are nowhere in the original, for example, and even Square doesn't shy away from taking butchered scripts and rewriting them with made-up additions to feel more "faithful" like in the case of FF6/SoM iOS English/FF4 PS1...).
Considering how the gameplay bits were butchered in the official localization, and the "re-translation" based on the Japanese vanilla rom included the missing stuff in English, it's not totally irredeemable for its time.
(and anyways FF4 GBA with a SNES sound restoration hack is the superior version)