You either need to do this:
<xsl:template match="teaserText_fr">
<xsl:copy>
<xsl:text disable-output-escaping="yes"><![CDATA[</xsl:text>
<xsl:copy-of select="*"/>
<xsl:text disable-output-escaping="yes">]]></xsl:text>
</xsl:copy>
</xsl:template>
Or this:
<xsl:template match="teaserText_fr">
<teaserText_fr>
<xsl:text disable-output-escaping="yes"><![CDATA[</xsl:text>
<xsl:copy-of select="*"/>
<xsl:text disable-output-escaping="yes">]]></xsl:text>
</teaserText_fr>
</xsl:template>
(I recommend the first approach)
and you should be all set.
To give the same treatment to any element whose name starts with “teaserText_”:
<xsl:template match="*[starts-with(local-name(), 'teaserText_')]">
<xsl:copy>
<xsl:text disable-output-escaping="yes"><![CDATA[</xsl:text>
<xsl:copy-of select="*"/>
<xsl:text disable-output-escaping="yes">]]></xsl:text>
</xsl:copy>
</xsl:template>