qmk_firmware/docs/reference_keymap_extras.md

Language-specific Keycodes

Keyboards are able to support a wide range of languages. However, this support is not actually achieved within the keyboard itself - instead, it sends numerical codes, which the operating system maps to the appropriate characters depending on the user's configured keyboard layout. By default (and per the HID spec), this is the US ANSI layout. For example, when a Swedish person presses the key with the å character printed on it, the keyboard is actually sending the keycode for [.

Obviously, this can get confusing, so QMK provides language-specific keycode aliases for many keyboard layouts. These won't do much on their own - you still have to set the matching keyboard layout in your OS settings. Think of them more as keycap labels for your keymap.

Simply #include one of the keycode headers below at the top of your keymap.c, and assign the keycodes defined in the header in place of the KC_ prefixed ones.

Sendstring Support

By default, SEND_STRING() assumes a US ANSI keyboard layout is set. If you are using a different layout, you can include one of the Sendstring LUT headers below in your keymap.c to override the lookup tables used for mapping ASCII characters to keycodes. You do not need to include the corresponding keymap_*.h header, as it is implicit when including the Sendstring header.

An important thing to note here is that SEND_STRING() only operates on ASCII text. This means that you cannot pass it a string containing Unicode characters - this unfortunately includes accented characters that may be present in your desired layout.
Many layouts make certain characters, such as Grave or Tilde, available only as dead keys, so you must add a space immediately after it in the string you want to send, to prevent it from potentially combining with the next character.
Certain other layouts have no Sendstring header as they do not use a Latin-derived alphabet (for example Greek and Russian), and thus there is no way to input most of the ASCII character set.

Header Files

These headers are located in quantum/keymap_extras/.

Layout	Keycodes Header	Sendstring LUT Header
Canadian Multilingual (CSA)	keymap_canadian_multilingual.h	sendstring_canadian_multilingual.h
Croatian	keymap_croatian.h	sendstring_croatian.h
Czech	keymap_czech.h	sendstring_czech.h
Danish	keymap_danish.h	sendstring_danish.h
Dutch (Belgium)	keymap_belgian.h	sendstring_belgian.h
English (Ireland)	keymap_irish.h
English (UK)	keymap_uk.h	sendstring_uk.h
English (US Extended)	keymap_us_extended.h
English (US International)	keymap_us_international.h	sendstring_us_international.h
English (US International, Linux)	keymap_us_international_linux.h
Estonian	keymap_estonian.h	sendstring_estonian.h
Finnish	keymap_finnish.h	sendstring_finnish.h
French	keymap_french.h	sendstring_french.h
French (AFNOR)	keymap_french_afnor.h	sendstring_french_afnor.h
French (BÉPO)	keymap_bepo.h	sendstring_bepo.h
French (Belgium)	keymap_belgian.h	sendstring_belgian.h
French (Switzerland)	keymap_swiss_fr.h	sendstring_swiss_fr.h
French (macOS, ISO)	keymap_french_mac_iso.h	sendstring_french_mac_iso.h
German	keymap_german.h	sendstring_german.h
German (Switzerland)	keymap_swiss_de.h	sendstring_swiss_de.h
German (macOS)	keymap_german_mac_iso.h	sendstring_german_mac_iso.h
German (Neo2)	keymap_neo2.h
Greek	keymap_greek.h
Hebrew	keymap_hebrew.h
Hungarian	keymap_hungarian.h	sendstring_hungarian.h
Icelandic	keymap_icelandic.h	sendstring_icelandic.h
Italian	keymap_italian.h	sendstring_italian.h
Italian (macOS, ANSI)	keymap_italian_mac_ansi.h	sendstring_italian_mac_ansi.h
Italian (macOS, ISO)	keymap_italian_mac_iso.h	sendstring_italian_mac_iso.h
Japanese	keymap_japanese.h	sendstring_japanese.h
Korean	keymap_korean.h
Latvian	keymap_latvian.h	sendstring_latvian.h
Lithuanian (ĄŽERTY)	keymap_lithuanian_azerty.h	sendstring_lithuanian_azerty.h
Lithuanian (QWERTY)	keymap_lithuanian_qwerty.h	sendstring_lithuanian_qwerty.h
Norwegian	keymap_norwegian.h	sendstring_norwegian.h
Polish	keymap_polish.h
Portuguese	keymap_portuguese.h	sendstring_portuguese.h
Portuguese (macOS, ISO)	keymap_portuguese_mac_iso.h	sendstring_portuguese_mac_iso.h
Portuguese (Brazil)	keymap_brazilian_abnt2.h	sendstring_brazilian_abnt2.h
Romanian	keymap_romanian.h	sendstring_romanian.h
Russian	keymap_russian.h
Serbian	keymap_serbian.h
Serbian (Latin)	keymap_serbian_latin.h	sendstring_serbian_latin.h
Slovak	keymap_slovak.h	sendstring_slovak.h
Slovenian	keymap_slovenian.h	sendstring_slovenian.h
Spanish	keymap_spanish.h	sendstring_spanish.h
Spanish (Dvorak)	keymap_spanish_dvorak.h	sendstring_spanish_dvorak.h
Swedish	keymap_swedish.h	sendstring_swedish.h
Swedish (macOS, ANSI)	keymap_swedish_mac_ansi.h
Swedish (macOS, ISO)	keymap_swedish_mac_iso.h
Swedish Pro (macOS, ANSI)	keymap_swedish_pro_mac_ansi.h
Swedish Pro (macOS, ISO)	keymap_swedish_pro_mac_iso.h
Turkish (F)	keymap_turkish_f.h	sendstring_turkish_f.h
Turkish (Q)	keymap_turkish_q.h	sendstring_turkish_q.h
Ukrainian	keymap_ukrainian.h

There are also a few which are not quite language-specific, but useful if you are not using a QWERTY layout:

Layout	Keycodes Header	Sendstring LUT Header
Colemak	keymap_colemak.h	sendstring_colemak.h
Dvorak	keymap_dvorak.h	sendstring_dvorak.h
Dvorak (French)	keymap_dvorak_fr.h	sendstring_dvorak_fr.h
Dvorak (Programmer)	keymap_dvorak_programmer.h	sendstring_dvorak_programmer.h
Norman	keymap_norman.h	sendstring_norman.h
Plover	keymap_plover.h
Plover (Dvorak)	keymap_plover_dvorak.h
Workman	keymap_workman.h	sendstring_workman.h
Workman (ZXCVM)	keymap_workman_zxcvm.h	sendstring_workman_zxcvm.h