alexander1970 started to cover things.
As with most things the answer is "it depends".
There are those people that will take games, typically in Japanese, and translate them to whatever they like. The process goes by a few names but ROM hacking or translation hacking are the main ones in English, though some will use the phrase fan translations.
For DS games specifically then you are probably on one of the main general purpose sites for such things, and for that matter it is probably also one of the main locations for GBA, Wii, Wii U, 3ds, Switch, and maybe gamecube.
https://gbatemp.net/forums/nds-rom-hacking-and-translations.41/ is the section in question for DS games. For Doraemon specifically I doubt you will find much. This is as games of properties known primarily for their anime don't tend to get translated, mostly as they are usually not very good and nobody wants to spend the hundreds of hours translating, and possibly fixing, bad anime games when there are all sorts of nice RPGs, puzzle games and whatever else that are still unplayable. If this is wrong though and the game is actually very fun (it happens from time to time) please do tell us why it is a good game and someone might take a look.
For older consoles
http://www.romhacking.net/ is considered the main hub for the ROM hacking world, and houses a serious library of translations.
There are other places out there, typically similar sites to this, and there are certain franchises (mario, mario kart, final fantasy, fire emblem, pokemon, advance wars and a few others to name but a few) that will also have extensively hacked games, which also means translated.
Bonus is if you are from Vietnam/speak primarily Vietnamese I have seen a few translations come out of there (if it is the only way games come in and you have some reasonable IT education, which Vietnam very much does
https://www.theregister.co.uk/2013/03/22/vietnam_kids_google_interview_pass/ , then you tend to see a fair few translations -- nobody releases games in China so there are usually hundreds of translations for various systems, often made before the game makes it from Japan to North America).
Afraid I don't have any good sites to link where discussion happens (if you find some please share as I like to keep on top of these things). However
http://www.romhacking.net/?page=tra...=20&order=&dir=&title=&author=&transsearch=Go
Should be a list of a few translations there. If you search for the authors, go on the links in the readmes and whatever else you might find more. For instance
https://nanivn.wordpress.com/ has not been updated in a few years but is a hacker that does translations to Vietnamese, and has some descriptions of file formats as well.
If a translation exists then they are usually not very hard to use. Most of the time you will need the ROM, the patch and the program to apply the patch, and obviously something to run it on after that. Patches tend to come with readmes though so go with that.
You might find prepatched versions out there in the world. Don't use them -- they might be patched badly, they might be an old patch, someone might have broken something, they might not have all the options and so on. They cause lots of trouble for ROM hackers as they go to try to help someone run the patched game and find all the issues were ones they sorted long ago, or would not have happened if someone patched it properly.
