How to reproduce it :

• into your skin, override captcha template with these params : placeholder-i18n, placeholder-catalogue and placeholder

  <xsl:call-template name="captcha">
                      <xsl:with-param name="key-name">captcha-key</xsl:with-param>
                      <xsl:with-param name="key-id">signup-<xsl:value-of select="$uniqueId"/>-captcha-key</xsl:with-param>
                      <xsl:with-param name="placeholder-i18n" select="true()"/>
                      <xsl:with-param name="placeholder">PLUGINS_WEB_SERVICE_USER_SIGNUP_VIEW_CAPTCHA</xsl:with-param>
                      <xsl:with-param name="placeholder-catalogue">plugin.web</xsl:with-param>

• the i18n key is not translated because template "captcha" calls the template "captcha_impl" without the placeholder-i18n neither the placeholder-catalogue.