This script should work with Seiken Densetsu, Final Fantasy Adventure, and Mystic Quest (English, French, or German).

In addition to simply changing the mapper and expanding the rom space (both of which are fairly trivial on Game Boy) the rom can be rearranged to allow for more space for scripts (including all the game's dialog) and more detailed map layouts.

Currently (2022-01-08) Mystic Editor does not have support for expanded roms.

Script Expansion:
In original SD/FFA/MQ script lines (the dialog in scenes) are embedded in tokenized scripts. Two of the game's 16 banks are dedicated entirely to scripts. The assembly access routine has been changed to  allow doubling the number of banks useable for scripts to four (with a 0x100 byte unusable "hole" in the fourth bank from 0x1600 to 0x16ff). Script banks must be kept contiguous. By default the sound bank is moved to make more space rather than moving the script banks.

I assume script expansion will be of the most interest to translators. Assuming you don't bother using the existing (fairly good) double byte encoding you will likely be able to fit more than 30,000 additional characters.

Map Expansion:
Maps have more than a few data structures. The highest level structure is at the beginning of bank 8 with one 11 byte entry for each of the 16 maps. It includes bank numbers and addresses for the structures that take up the most space and which store (mildly compressed) background maps for every map screen. The fact that this involves compression rather than fixed sizes for each screen's background ended up being a problem for Ember of Mana (a rom hack in progress).

In original SD/FFA/MQ banks 5, 6, and 7 are dedicated entirely to these particular structures. While there is technically no constraint requiring that the banks for this be contiguous, keeping them together seems smart for editing purposes so this allows moving the existing banks somewhere with room to grow.

This does not currently allow for additional tiles or larger maps.

There's more information on options and what this does in the script its self.
