Within the script editor folder, go to Atlas, since that’s the insertion program. You’ll need to provide your own rom. Name it FE1Eng.nes if you don’t feel like fiddling with the batch file.
For FE2, I’ve dumped the text and it can be found on a git repo: FE2 Text Dump
However, I have not configured Atlas for FE2 yet. Cartographer dumps go well enough with Atlas insertion, but they need a bit of editing.
As for adding extra letters, both FE1 and FE2 use extra font space for “squished tiles”, which serve to reduce the amount of rom and screen space text takes. FE2 makes more efficient use of them. Thankfully, editing this is easy. Go to 0x40010 in both games in your tile editor of choice (mine’s YY-CHR) and edit away. You’ll need to adjust the Atlas table file when you’re done: this is just a matter of replacing “30 = io” with “30 = Ö”, assuming you put that particular letter there.