Hi Steve and Maria,
this is way for advanced users (I did it this way in past as well), but for "every day" users is there an button "Change UI Language" that helps changing interface language.
The interface of SignPuddle is separate from the puddles contents.
If you used this link, you'd see the Czech interface:
But it you used this link, you'd see the English interface:
So if you ever find you can't understand the use interface, find the "ui=" section in the url and change it to "ui=1" and reload the page.