How would the keyboard support diacritic marks? USB HID compliant keyboards send *codes* and it's OS' job to interpret them. The firmware should only support sending the ISO extra key's code.
To be entirely technical, the firmware sends the codes, and the firmware is programmable
Will you include an option to support a nordic layout with åäö keys?
For the keyboards that are fully assembled, I can write and program a custom layout, but that part is up to you guys - I don't have any experience with nordic languages or layouts. I remember /u/ramnes mentioning International Qwerty was a good alternative. You'll have four whole layers to mess with (no mods, left mod, right mod, both mods), in addition to any other mods you may want (other than the two I'm using). Feel free to post layouts you have in mind here, and I can give advice on what I think will work the best.
Just curious (I apologize, if it's a stupid question, I know next to nothing about PCB design), in case I decide to switch from The Grid to MIT, which has one key less, I won't have to deal with diodes at all? What about the firmware?
All the diodes can stay in place for switch configurations, thanks to fancy engineering. Because of this, the firmware can also stay the same, but you may want to reprogram it since you're losing/gaining a key. Desoldering isn't the easiest thing in the world, but with some practice and online videos, I'm pretty sure anybody could figure it out.