I am trying to produce an A4 page for our doors. I have used LuaLaTeX. In the spirit of exploration, I tried to use \begin{czech} and \begin{english} from polyglossia package. The MWE is this:
\documentclass[12pt,a4paper,landscape,oneside]{minimal}
\usepackage[margin=2.5cm]{geometry}
\usepackage{fontspec}
\usepackage{polyglossia}
\setdefaultlanguage{czech}
\setotherlanguage{english}
\begin{document}
\begin{center}
\begin{czech}
\fontsize{1.5cm}{1em}\selectfont
\textsc{Až projdete, \textbf{otočte} se a ujistěte se, že jsou \textbf{dveře \underline{úplně zavřené}}.} \\
\end{czech}
\hrule
\begin{english}
\fontsize{1.5cm}{1em}\selectfont
\textsc{After you walk through, \textbf{turn around} and make sure the \textbf{door is \underline{completely closed}}.}
\end{english}
\end{center}
\end{document}
The resulting document looks like this:

Notice how the line spacing is different for the English message part. Commenting out the polyglossia language selections fixes the result for me.
Why is using the \begin{english} breaking the line spacing and how can I avoid it / fix it?
UPDATE
It appears that 1em is not accepted format of the size for the \fontsize macro. Using 1.8cm works and fixes the issue.

\fontsize{1.5cm}{1.8cm}\selectfont. And add a\parbefore closing the language environments. The czech is faulty too but there the accents forces the lines more apart. – Ulrike Fischer Dec 09 '14 at 12:37english. Leave the\fontsizeand the exglish text and you will see the same thing. Not sure why, but it is a bit odd to have a line spacing that is smaller than the font size. – daleif Dec 09 '14 at 12:38