I think I found the problem. I edited the name in es_systems.cfg famicom isn't a core so that's why it wouldn't launch. Even if the gui show you the options to configure a famicom.core somehow it doesn't exist. So the <name></name> tag need to be named nes. Same applies for super famicom.
<name>nes</name>
I guess if I could find the core folder I might be able to copy that folder and have a new famicom core.
So in short to add famicom/super famicom to show in your US/EUR roms collection you need to:
edit your /recalbox/share/system/.emulationstation/es_systems.cfg
<system>
<fullname>Family Computer</fullname>
<name>nes</name>
<path>/recalbox/share/roms/famicom</path>
<extension>.nes .NES .zip .ZIP</extension>
<command>python /usr/lib/python2.7/site-packages/configgen/emulatorlauncher.pyc %CONTROLLERSCONFIG% -system %SYSTEM% -rom %ROM% -emulator %EMULATOR% -core %CORE% -ratio %RATIO%</command>
<platform>nes</platform>
<theme>famicom</theme>
<emulators>
<emulator name="libretro">
<cores>
<core>fceunext</core>
<core>quicknes</core>
<core>fceumm</core>
<core>nestopia</core>
</cores>
</emulator>
</emulators>
</system>
copy nes folder to /recalbox/share/system/.emulationstation/themes/recalbox-next/famicom
copy the svg art from jp to famicom/data directory
delete us/eu/jp sub folders
edit custom.xml and remove region section at the bottom of code
Last two steps might ot be needed but if you like to keep the code neat and clean you can do it.
Imgur
