[SalesForce] How to display percent values in lightning component

Both will work. It seems to me as some syntax errors;

<aura:component >
    <aura:attribute name="myList" type="List" />

    <!-- What I want 1: iterate -->
    <aura:iteration var="listItem" items="{!v.myList}" indexVar="index">
        //{!v.listItem}
        {!listItem}//Change: This is not an attribute. So 'v.' is not required.
    </aura:iteration>

    <!-- What I want 2: Outside of the iteration, get some elements by index -->
    //{!v.listItem[2]}
    {!v.myList[2]}//Change: 'myList' is the original attribute. 'listItem' is just local to 'aura:iteration'(in this case)

</aura:component>

For client to apex methods/variable you need @AuraEnabled annotation AuraEnabled Annotation

You may have to annotate every property you are accessing in your component with @AuraEnabled.

The @AuraEnabled annotation enables client- and server-side access to an Apex controller method. https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_classes_annotation_AuraEnabled.htm

Component:

<aura:component controller="testmecontroller5parser" implements="force:appHostable,flexipage:availableForAllPageTypes,flexipage:availableForRecordHome,force:hasRecordId,force:lightningQuickAction" access="global" >
    <aura:handler name="init" value="{!this}" action="{!c.doInit}"/>
    <aura:attribute type="List" name="Interlocuteurs"/>
    hello: {!v.Interlocuteurs}
    <aura:iteration items="{!v.Interlocuteurs}" var="Interl">
        hello : {!Interl.contact}
    </aura:iteration>
</aura:component>

Controller:

({
    doInit : function(cmp, event, helper) {
        var action = cmp.get("c.getInterlocuteurs");

        action.setCallback(this, function(response) {
            var state = response.getState();
            if (state === "SUCCESS") {
                cmp.set('v.Interlocuteurs', response.getReturnValue());
            console.log(cmp.get('v.Interlocuteurs'));
            }

        });
        $A.enqueueAction(action);
}

})

Apex controller:

public class testmecontroller5parser {

    public class testmecontroller5 {
        @AuraEnabled
        public String contact;
        @AuraEnabled
        public String postalcode;
        @AuraEnabled
        public String contactnumber;
    }

    @AuraEnabled
    public static List<testmecontroller5> getInterlocuteurs(){
        List<testmecontroller5> Interlocuteurs = new List<testmecontroller5>();
        testmecontroller5 tst = new testmecontroller5();
        tst.contact = 'contact 1';
        Interlocuteurs.add(tst);
        return Interlocuteurs;
    }
}

Output: