Mi rispondo da solo per chi ne avesse bisogno in futuro.
Bisogna imparare a lavorare con java.
Per esempio, ho risolto il mio problema con la data semplicemente passando, in iReport, dal designer alla visualizzazione del codice xml. Il campo a cui si riferiva la data era impostato su String. Ho cambiato questa impostazione in Date ed ho importato la classe java.util.Date. Poi con il pattern (la formattazione di campo) ho scelto il formato data desiderato. Di seguit il codice:
<?xml version="1.0" encoding="UTF-8"?>
<jasperReport xmlns="
jasperreports.sourceforge.net/jasperreports" xmlns:xsi="
www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="
jasperreports.sourceforge.net/jasperreports jasperreports.sourceforge.net/xsd/jasperreport.xsd" name="Fattura4" pageWidth="595" pageHeight="842" columnWidth="555" leftMargin="20" rightMargin="20" topMargin="20" bottomMargin="20">
<property name="ireport.zoom" value="1.0"/>
<property name="ireport.x" value="0"/>
<property name="ireport.y" value="0"/>
<import value="java.util.Date"/>
<queryString language="xPath">
<![CDATA[]]>
</queryString>
<field name="Data_Fattura-date_invoice"
class="java.util.Date">
<fieldDescription><![CDATA[/data/record/Data_Fattura-date_invoice]]></fieldDescription>
</field>
<field name="Nazione-country" class="java.lang.String">
<fieldDescription><![CDATA[/data/record/Commercial_Entity-commercial_partner_id/Nazione-country]]></fieldDescription>
</field>
<field name="Nazione-country_id" class="java.lang.String">
<fieldDescription><![CDATA[/data/record/Commercial_Entity-commercial_partner_id/Nazione-country_id]]></fieldDescription>
</field>
<background>
<band splitType="Stretch"/>
</background>
<title>
<band height="79" splitType="Stretch"/>
</title>
<pageHeader>
<band height="35" splitType="Stretch"/>
</pageHeader>
<columnHeader>
<band height="61" splitType="Stretch">
<textField
pattern="EEEEE dd MMMMM yyyy">
<reportElement x="0" y="0" width="315" height="20"/>
<textElement lineSpacing="Single"/>
<textFieldExpression class="
java.util.Date"><![CDATA[$F{Data_Fattura-date_invoice}]]></textFieldExpression>
</textField>
</band>
</columnHeader>
<detail>
<band height="125" splitType="Stretch">
<textField>
<reportElement x="0" y="2" width="281" height="20"/>
<textElement lineSpacing="Single"/>
<textFieldExpression class="java.lang.String"><![CDATA[$F{Nazione-country}]]></textFieldExpression>
</textField>
<staticText>
<reportElement x="341" y="2" width="214" height="20"/>
<textElement lineSpacing="Single"/>
<text><![CDATA[Nazione]]></text>
</staticText>
<textField isBlankWhenNull="false">
<reportElement x="0" y="44" width="281" height="20"/>
<textElement lineSpacing="Single"/>
<textFieldExpression class="java.lang.String"><![CDATA[$F{Nazione-country_id}]]></textFieldExpression>
</textField>
<staticText>
<reportElement x="341" y="44" width="214" height="20"/>
<textElement lineSpacing="Single"/>
<text><![CDATA[Nazione_id]]></text>
</staticText>
</band>
</detail>
<columnFooter>
<band height="45" splitType="Stretch"/>
</columnFooter>
<pageFooter>
<band height="54" splitType="Stretch"/>
</pageFooter>
<summary>
<band height="42" splitType="Stretch"/>
</summary>
</jasperReport>