Actual result. A input field of type number without any values set (see attached Screenshot)

Can't confirm with Joomla 3.9.27-dev, PHP7.4, Protostar, with or without setlocale, FireFox:

<!-- Body -->
 <div class="body" id="top">
  <div class="container<?php echo ($params->get('fluidContainer') ? '-fluid' : ''); ?>">
<?php
// setlocale(LC_ALL, 'de_DE');

$array= array('value' => "34.3", 'autocomplete' => false, 'disabled' => false, 'hint' => '', 'required' => false, 'autofocus' => false, 'readonly' => false );

echo JLayoutHelper::render('joomla.form.field.number', $array);
?>
 <!-- Header -->

I have the same problem with the comma instead of the point on Joomla 3.9.26 and 3.9.27-devdev.

In two different hosting companies, two different Joomla's, after entering setlocale(LC_ALL, 'pl_PL');, I have commas instead of the point.

I've tested only:

A input field of type number without any values set (see attached Screenshot)`

I have commas instead of the point.

That's what I expect on a (my) German website. And I didn't get any "invalid" hint with Firefox when I left the field via Tab.

Therefore you must describe the issue/testing instructions more precisely.

I noticed this problem in RSForm and reCAPTCHA V3 plugin on client hosting.
Everything worked fine on my hosting.
I started looking for a solution to the problem with the plug-in manufacturer and then in hosting.
Hosting told me to add setlocale (LC_ALL, 'en_US.utf8'); to index.php and it worked.

Then I started looking for a problem with Joomla setlocale and that's how I ended up here.

Now I tested according to your instructions in the Protostar template and it does not work in Chrome, Opera, Safari, Brave (blank field).
Only works in Firefox (apparently accepts commas in fractions).

I can reproduce this on Windows by using:
setlocale(LC_ALL, 'Polish');

On a shared hosting environment (CentOS) I can reproduce this with:
setlocale(LC_ALL, 'pl_PL');

This happens on PHP < 8 only. With PHP 8+ there are some changes documented here:
https://php.watch/versions/8.0/float-to-string-locale-independent
Which is in my opinion a feature that has been fixed: PHP shouldn't globally change and localize output on its own. I don't really see a way to fix this in Joomla! at this point

This happens on PHP < 8 only.

I can confirm your find in some other browsers than Firefox. PHP < 8. Whenever a setlocale is used in my example above the field is empty. PHP8: all fine.

On the other hand I don't need a setlocale. Any browser shows me a comma instead of a dot on a German site. But maybe I'm missing something. Saving does not work or whatever?

Forms with fractions do not work on some shared servers in Poland.!
(https://issues.joomla.org/uploads/1/d1dfa4148ee221ea207f08ceed0106f1.png)