Incorrect font size in tag cloud on decimal comma locales

[SergeyBiryukov]

On ​some PHP locales, comma is used as a decimal separator.

This creates incorrect font sizes in tag cloud (e.g. the one on Install Plugins screen). See the screenshot.

Browser fails to handle font-size attributes with commas properly, as CSS specification only supports decimal point as a separator:
​http://www.w3.org/TR/CSS21/syndata.html#length-units

To reproduce, add this to wp-config.php:

setlocale(LC_ALL, 'fr_FR');

Related: #18460, #19593

[nacin]

#19593