> The 'for in' looks like xslt2 function.
well an xpath2 expression, yes.

> for $x in key('tr_string-by-str', $key) return (x$/ru, $x/en, .)[1]

but you are in luck here as the for isn't actually doing anything in
that case you could write
key('tr_string-by-str', $key)/(ru,en, current())[1]
in xpath2
or in xslt1 
<xsl:variable nae="x" select="key('tr_string-by-str', $key)"/>
<xsl:choose>
 <xsl:when test="$x/ru">
  <xsl:value-of select="$x/ru"/>
 </xsl:when>
 <xsl:when test="$x/en">
  <xsl:value-of select="$x/en"/>
 </xsl:when>
...

david

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