Use "concatenatable" language codes

Marlin/Configuration.h

Marlin/src/core/language.h

Marlin/src/lcd/dogm/fontdata/langdata_el_gr.h

Marlin/src/lcd/dogm/fontdata/langdata_jp_kana.h

Marlin/src/lcd/dogm/fontdata/langdata_pt_br.h

Marlin/src/lcd/language/language_el_gr.h

Marlin/src/lcd/language/language_jp_kana.h

Marlin/src/lcd/language/language_pt_br.h

buildroot/share/fonts/genallfont.sh

buildroot/share/tests/megaatmega2560-tests

config/default/Configuration.h

config/examples/3DFabXYZ/Migbot/Configuration.h

config/examples/ADIMLab/Gantry v1/Configuration.h

config/examples/ADIMLab/Gantry v2/Configuration.h

config/examples/AlephObjects/TAZ4/Configuration.h

config/examples/Alfawise/U20-bltouch/Configuration.h

config/examples/Alfawise/U20/Configuration.h

config/examples/AliExpress/CL-260/Configuration.h

config/examples/AliExpress/UM2pExt/Configuration.h

config/examples/Anet/A2/Configuration.h