When you put it that way - I recall the installer log I created while troubleshooting another issue related to grub.
I remember the log listed something about parsing the selected locale name and picking the keymap from the second part e.g. en_AU would make au.
The log - https://0x0.st/KOXd.txt - now I am doubting my memory - it seems to be wrong - could be this section I am remembering ?
10:16:04 [6]: void CalamaresApplication::init()
Calamares version: 3.4.0
.. Using Qt version: 6.10.0
.. Build type: Debug
.. Using PyBind11
.. Using settings: "/etc/calamares/settings.conf"
.. Using log file: "/root/.cache/calamares/session.log"
.. Languages: QList("ar", "as", "ast", "az", "az_AZ", "be", "bg", "bn", "ca", "ca@valencia", "cs_CZ", "da", "de", "el", "en", "en_GB", "eo", "es", "es_AR", "es_MX", "et", "eu", "fa", "fi_FI", "fr", "fur", "gl", "he", "hi", "hr", "hu", "ia", "id", "is", "it_IT", "ja", "ka", "ko", "lt", "ml", "mr", "nb", "nl", "oc", "pl", "pt_BR", "pt_PT", "ro", "ru", "si", "sk", "sl", "sq", "sr", "sr@latin", "sv", "tg", "th", "tr_TR", "uk", "uz", "vi", "zh_CN", "zh_TW")
10:16:11 [6]: static LocaleConfiguration LocaleConfiguration::fromLanguageAndLocation(const QString&, const QStringList&, const QString&)
Mapping "en" in "DK" to locale.
.. Got best match for "en" as "en_AU.UTF-8"
.. Got best match for "en_DK" as "en_DK.UTF-8"
10:16:11 [6]: static LocaleConfiguration LocaleConfiguration::fromLanguageAndLocation(const QString&, const QStringList&, const QString&)
Mapping "en" in "DK" to locale.
.. Got best match for "en" as "en_AU.UTF-8"
.. Got best match for "en_DK" as "en_DK.UTF-8"
10:16:11 [6]: static LocaleConfiguration LocaleConfiguration::fromLanguageAndLocation(const QString&, const QStringList&, const QString&)
Mapping "en" in "DK" to locale.
.. Got best match for "en" as "en_AU.UTF-8"
.. Got best match for "en_DK" as "en_DK.UTF-8"
10:16:12 [6]: void Config::guessLocaleKeyboardLayout()
Got locale language "en_DK.UTF-8"
.. looking for locale part "DK"
.. matched "dk"
I suggest you create an issue at Calamares/calamares: Distribution-independent installer framework - Codeberg.org - as for your question you should simply change to en.
I shared the log on 0x0.st and linked it in the issue - will try to locate the issue and link it.
If your system is Plasma based - it might have a ~/.local/plasma-localerc which rewrites the locale.