While I'm not personally familiar with this game or what specific technical obstacles it has -- though given the platform and the genre alone, I'd guess there are a lot -- I'd like to point out that on the PCE, adding subtitles isn't really "simpler" than dubbing. In fact, considered solely in terms of raw hacking work required, adding subtitles is much more difficult than "just" replacing the audio. Despite what the graphics might make you believe, the PCE is ultimately an 8-bit system from 1987, and it provides exactly one layer of background graphics and one layer of sprites. If you want subtitles, the only viable option in most situations is to add them in as a sprite overlay, which gets complicated very quickly: you have to find space in VRAM to load the graphics for the text, rework the sprite table generator routines so your subtitle sprites show up where needed (and appear on top of everything else in the scene), and try to achieve all this while somehow not disrupting the existing visuals. And keep in mind there's an inviolable limit of 16 sprites per line and 64 sprites total, so there are plenty of situations where you just plain can't add subtitles without destructively editing the scene in some way (e.g. letterboxing).
Of course, producing a quality dub has its own set of complications that make it at least as bad as subtitling, since on top of having to organize and direct a bunch of voice performances, you're going to need to alter the cutscenes to extend the length of certain lines, redo the lip sync, etc. But it's not like you can just magically slap subtitles on instead and call it a day. I don't know what the exact circumstances of this project are, but if a lot of work has already gone into it with the specific goal of producing a dub, trying to switch to subtitles at this point would probably mean starting almost from scratch. It's very unlikely that it would make the translation come out any faster.
I can empathize with anyone trying to put together a translation for this crazy little machine, so best of luck to everyone involved with the project. The world really needs more PCECD translations.