Calc guard error message displays in wrong translation

[janherman]

Describe the bug
I discovered if I fill wrong result in calc guard field, calc guard shows error message in English, even though my site is in Czech. This is the relevant part of the controller:

$form = new Form([
     // other rules ...
    "captcha" => [
        "rules" => ["required", "num"],
        "message" => "Vyplňte pole"
    ],
]);

This is the template:

<label>
    Vypňte výsledek příkladu <?= uniform_captcha() ?> =
    <?= captcha_field("", "contact-form__field"); ?>
</label>
<?php if(array_key_exists("captcha", $form->errors())): ?>
	<p class="contact-form__error"><?= $form->errors()["captcha"][0] ?></p>
<?php endif; ?>

Now, when I leave the captcha field blank, the returned error message is as defined in new Form().

But when I fill wrong result (let's say 3 + 3 = 77), English error message 'Please solve the arithmetic problem.' is returned.

Expected behavior
Czech error message should be shown.

Additional context
I tried it in Kirby 4.6.1 plainkit with the latest Uniform.

[janherman]

I found what causes this issue. It actually is Kirby itself. My site is single-language and the language is not English, which is default in Kirby. Turns out Kirby does not handle locale settings well on single-language pages. There is even a suggestion addressing this behavior on kirby.nolt.io