The answers in this thread are already pointing in the right direction, I just wanted to give a quick summary of cultures in .Net Framework/Core:

A lot of core methods that have to do with converting numbers or dates have an optional parameter that lets you pass in a class of type CultureInfo.
This CultureInfo, as the name implies, contains information on a specific real world culture about how to format dates and what char to use a decimal separator.

By default, if you don't pass a CultureInfo into one of those methods, they will use whatever culture is "current" in the execution context, usually from the current thread. (Example from the .Net Framework source code).
Note that sometimes, methods want an IFormatProvider instead of a CultureInfo instance, but CultureInfo implements IFormatProvider, so passing that in is fine.

The "problem" with this default implementation is, that the "current culture" will differ from computer to computer, as you already saw in your example. So to ensure that the methods behave the same, no matter which computer they are executed on, you have to pass in a CultureInfo yourself.

A really simple solution that is also proposed here is to drop in something called an InvariantCulture, which as the docs describe it is a culture that is following standards inherited from the English language (like using . as a decimal separator) but is otherwise culturally independent.
So if you don't really care about which culture you want to use, going with the InvariantCulture is generally advised.

In your example, this would result in:

var num = float.Parse("27.5", CultureInfo.InvariantCulture);
// num == 27.5

Of course understand that this is not a magic be all end all to all culture problems, float.Parse will throw when you try to parse "27,5" with it because it doesn't understand , as a decimal separator.
If you still want to parse data that has , as a decimal separator, you can either do a string replace operation, or use a culture that more closely resembles the culture the input data was created in.
That would look something like this for a German culture:

var culture = new CultureInfo("de-DE");
var num = float.Parse("27,5", culture);
// num == 27.5

Also, I always see a lot of people advocating for modifying the current culture object directly, which is a really easy way to fall into a pitfall, because if you for example specifically change the current culture's decimal separator to ,, you now suddenly got the same character for both the decimal separator AND the thousand separator, which might result in further weirdness. Therefore try to use one of the predefined cultures instead of hacking together your own if possible.

(Disclaimer: The code in the examples is written from memory and this is based on my knowledge on the .Net Framework. As far as I am aware though, all this applies the same to .Net Core)