Avoid line feeds before and after <lang:string/>. Your sample markup should read like this:
<xsl:variable name="SendText">
<xsl:text><lang:string key="Resource, Resources.FormResource.ContactSendLabel" /></xsl:text>
</xsl:variable>
<input name="submitContactForm" id="submitContactForm" class="button" type="submit" value="{$SendText}" />