Error when params are in Xpath expression

<!-- get translation of a string -->
    <xsl:template name="translateString">
        <xsl:param name="str" select="'recent_albums'"/>
        <xsl:param name="toLang"/>
        <xsl:param name="fallback" select="'en'"/>
		
        <xsl:choose>
            <xsl:when test="true()">

				<xsl:value-of
select="document('')/*/trans:tr/tr_string[@str=$str]/$toLang"/>
            </xsl:when>
            <xsl:otherwise>
                <xsl:value-of select="'not found'"/>
            </xsl:otherwise>
        </xsl:choose>
    </xsl:template>


I get errors when running this template.
If I manually replace the $str and $toLang, then I am getting the
correct result.

<xsl:value-of select="document('')/*/trans:tr/tr_string[@str='recent_images']/ru"/>

What is wrong with my syntax that uses params? Are params allowed in
Xpath expression?