qmk-firmware/docs/reference_keymap_extras.md

Language-specific Keycodes

Keyboards are able to support a wide range of languages. However, they do not send the actual characters produced by pressing their keys - instead, they send numerical codes. In the USB HID spec, these are called "usages", although they are more often referred to as "scancodes" or "keycodes" when in the context of keyboards. Less than 256 usages are defined in the HID Keyboard/Keypad usage page, and some of those do nothing on modern operating systems. So, how is this language support achieved?

In a nutshell, the operating system maps the usages it receives to the appropriate character based on the user's configured keyboard 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 could 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.

To use these, simply #include the corresponding header file in your keymap.c, and add the keycodes defined in them in place of the KC_ prefixed ones:

Layout	Header
Canadian Multilingual (CSA)	keymap_canadian_multilingual.h
Croatian	keymap_croatian.h
Czech	keymap_czech.h
Danish	keymap_danish.h
Dutch (Belgium)	keymap_belgian.h
English (Ireland)	keymap_irish.h
English (UK)	keymap_uk.h
English (US International)	keymap_us_international.h
Estonian	keymap_estonian.h
Finnish	keymap_finnish.h
French	keymap_french.h
French (BÉPO)	keymap_bepo.h
French (Belgium)	keymap_belgian.h
French (Switzerland)	keymap_fr_ch.h
French (macOS, ISO)	keymap_french_osx.h
German	keymap_german.h
German (Switzerland)	keymap_german_ch.h
German (macOS)	keymap_german_osx.h
German (Neo2)*	keymap_neo2.h
Greek*	keymap_greek.h
Hebrew*	keymap_hebrew.h
Hungarian	keymap_hungarian.h
Icelandic	keymap_icelandic.h
Italian	keymap_italian.h
Italian (macOS, ANSI)	keymap_italian_osx_ansi.h
Italian (macOS, ISO)	keymap_italian_osx_iso.h
Japanese	keymap_jp.h
Korean	keymap_korean.h
Latvian	keymap_latvian.h
Lithuanian (ĄŽERTY)	keymap_lithuanian_azerty.h
Lithuanian (QWERTY)	keymap_lithuanian_qwerty.h
Norwegian	keymap_norwegian.h
Polish	keymap_polish.h
Portuguese	keymap_portuguese.h
Portuguese (Brazil)	keymap_br_abnt2.h
Romanian	keymap_romanian.h
Russian*	keymap_russian.h
Serbian*	keymap_serbian.h
Serbian (Latin)	keymap_serbian_latin.h
Slovak	keymap_slovak.h
Slovenian	keymap_slovenian.h
Spanish	keymap_spanish.h
Spanish (Dvorak)	keymap_spanish_dvorak.h
Swedish	keymap_swedish.h
Turkish (F)	keymap_turkish_f.h
Turkish (Q)	keymap_turkish_q.h

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

Layout	Header
Colemak	keymap_colemak.h
Dvorak	keymap_dvorak.h
Dvorak (Programmer)	keymap_dvp.h
Norman	keymap_norman.h
Plover*	keymap_plover.h
Plover (Dvorak)*	keymap_plover_dvorak.h
Steno*	keymap_steno.h
Workman	keymap_workman.h
Workman (ZXCVM)	keymap_workman_zxcvm.h

Sendstring Support

By default, SEND_STRING() assumes a US ANSI keyboard layout is set. If you are using a different layout, you can also #include "sendstring_*.h" (as above) in your keymap to override the lookup tables used for mapping ASCII characters to keycodes.

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. These are marked above with a *.