Mother 3 is a GBA game, so isn't extra space basically free on that platform ? Unless the original game already uses the maximum allowed by the platform (I think that's 64 MB ?) but games using that much space were rare, most being 8 or 16 MB.
Also you should definitely contact the team doing the english translation - they'll most probably gladly help you. Don't worry about the extra diactrics, typically they're a lesser problem into the whole mess - especially on a flexible modern platform like the GBA (if it was the NES you'd be talking about it could be a problem). Actually translating the script is a much larger problem. Also one problem with slavic languages is that surnames have to be declined, in particular names the players give to their characters.
If you go by the space allowed if you use the full cart address range then it is 32 megabytes. Flash carts and a few of the video carts pushed this higher by a few means (bankswitching/page setting mostly). https://mgba.io/2015/10/20/dumping-the-undumped/
for the video stuff. Flash carts were quite boring in the end, give or take the pogoshell stuff and maybe the EZ3 stuff which got used for the PCE-CD emulation.
For most practical purposes emulators and flash carts though will be limited to said 32 megs -- I don't know if mgba is still the only thing with support for the higher size video carts but I am not expecting anything hardware wise to handle it any time soon, and legacy emulators/emulators for non PC devices I also don't count on supporting it.
Yes that space is basically free (as hard as it gets to use it is the various waitstate mirrors, and having to either add one or use 09 at the front rather than 08 for the most common waitstate/read style*) and without most of the issues that plague those playing with the NES and SNES trying to do ROM expansion.
*... that is really confusing now I have written it. The most common place to read a GBA cart is in the 08000000 through 09FFFFFF window, and not all that much will copy things from the ROM to the RAM to read as the ROM is so fast. As most games are 16 megabytes or less this means all the reads, and thus all the pointers, are of the form 08??????, hence the rule of thumb of just chuck a 08 in front of the address in the ROM, search for 08 for pointers... for the upper 16 megs then this is 09??????, or add 01 to the 08. Waitstate mirrors is much as it sounds and different GBA read locations have different priorities, which can be changed by the code being run (I don't know that I have ever seen any in the wild but I never went looking). For the terminally bored http://problemkaputt.de/gbatek.htm#gbasystemcontrol
Mother 3 though is one of the rare games ( http://gbatemp.net/threads/buying-a-gba-flash-cart-in-2013.341203/page-18#post-4756995
) to use the upper 16 meg range though (if you play with flash carts you will usually see it referred to as the 256Mbit titles), and beyond that comments from the devs during the translation gave me the impression that unlike most of the games playing in the upper range that Mother 3 used pretty much everything and thus forces you to reclaim things as you go. I don't know if it could quite claim the crown of the GBA's Final Fantasy 3 in terms of space used but that is where I usually head.
Other games I have looked at in that size range** usually only make use of some 20 something megs which still gives you megabytes to play with (more than enough for text and most other purposes). There will be a few hacks that use all the space as well (Golden Sun voice acting experiment probably being a good one to look at here) but that is getting off topic.
It is not the only time people are pressed for space on the GBA -- said flash carts often were made with the 16 megs is most common thing in mind and thus the greater than 16 megs stuff can be more annoying to run and thus it is desirable to keep it below it if you can. However at all points you can still say "poke it" and use the upper region.
If the Mother 3 peeps released tools though then that makes life rather easier. I must have missed/forgotten about those being dropped.
**several had a couple of bytes at the end that was not 00 or FF which troubled certain cheat programs, reset patching, trimming programs (useful if your carts are 32 megabytes in size a lot of the time). Prior to those few bytes though was a large blank area and thus with a tweak could be trimmed, though I later heard I think it was Riviera might have broken in some subtle ways if you did that, whether as a kind of anti piracy or not I don't know.