Style for wrapped text using a table

I create my table using xslt and want a cell with wrapped text (or better: text with a line break in it).

So my xslt generates the following XML for sheets:

<sheetData>
  <row r="1">
    <c r="A1" t="inlineStr">
      <is>
        <t>
          a simple string
          line break jeeeehaaa
        </t>
      </is>
    </c>
  </row>
</sheetData>

After I finished creating my book, I will open it in Excel; and there is no line break, just a simple single line line in A1.