Locale

The locale sets the language for program messages and the formats for dates, numbers, money and sorting. For example en_US.UTF-8 means English, United States formats, UTF-8 text.

Locale	date +%A	Number
en_US.UTF-8	Tuesday	1,234.5
fi_FI.UTF-8	tiistai	1 234,5
ja_JP.UTF-8	火曜日	1,234.5
C.UTF-8	Tuesday	1234.5

How it’s set

1. The installer asks for a locale (default en_US.UTF-8) — or use --locale=….

2. It makes sure the locale exists on the system, generating or installing it if needed.

3. It saves your choice in ~/.config/locale.conf:

   LANG=fi_FI.UTF-8

4. Every shell start, .zshenv (zsh) and .bash_profile (bash) read that file and set LANG and LC_ALL.

Change your mind later? Edit ~/.config/locale.conf and open a new shell — or re-run the installer with --locale=… to also install it.

No locale.conf? The shells use en_US.UTF-8.

When the locale isn’t installed

The shells first check locale -a (the list of installed locales). If your locale isn’t there, they use C.UTF-8 instead.

Why: forcing a locale that doesn’t exist makes bash print setlocale: cannot change locale on every start, and Perl, Python and others print “Setting locale failed” on every run. C.UTF-8 exists almost everywhere, keeps UTF-8 (emoji, accents) working, and shows plain-English messages.

The check costs about 1–2 ms per shell start.

How the installer adds a locale

System	What it does
Ubuntu, Debian	installs locales, enables the line in /etc/locale.gen, runs locale-gen
Arch	enables the line in /etc/locale.gen, runs locale-gen
Fedora, RHEL	installs a language pack, e.g. glibc-langpack-en for en_*
openSUSE	installs glibc-locale (all locales in one package)
Alpine	nothing — musl doesn’t use compiled locales
macOS	nothing — every locale is preinstalled

locale-gen builds a locale from its source description so programs can use it. Enabling it in /etc/locale.gen (not just generating it once) means it survives later rebuilds, e.g. after a glibc update.

Edge cases

• Locale names are flexible: ja_JP, ja_JP.utf8 and ja_JP.UTF-8 all mean the same thing. The installer always stores .UTF-8 — the configs assume UTF-8.
• Arch Docker image: only a few locales (including en_US) can be generated. The image deletes the others’ source files to save space (NoExtract in /etc/pacman.conf). A real Arch install has all of them. The installer warns and the shells fall back to C.UTF-8.
• LC_ALL is set, not just LANG. LC_ALL overrides every other locale setting. That’s unusual (normally only LANG is set) but Fastlane and CocoaPods need a UTF-8 LC_ALL. If you want, say, English messages with Finnish date formats, LC_ALL gets in the way — unset LC_ALL in ~/.zshenv.local and set LC_TIME=fi_FI.UTF-8 there.
• Linux desktops may also read ~/.config/locale.conf (Arch’s /etc/profile.d/locale.sh does), so the choice can affect more than your shell there.