Dynamic Email Templates

1 - DynamicEmailController.class

public class DynamicEmailController {
    
    public DynamicEmailController() {}
    
    public String emailRelatedToId {get; set;}
    public String emailTemplateId {get; set;}
    public Boolean isHtml {get; set;}
    
    private Messaging.SingleEmailMessage renderedEmail {
        get {
            if (renderedEmail==null) {
                renderedEmail = Messaging.renderStoredEmailTemplate(emailTemplateId, '', emailRelatedToId);
            }
            return renderedEmail;
        }
        set;
    }
    
    public String getMergedEmailHtmlBody() {
        return renderedEmail.getHtmlBody();
    }
    
    public String getMergedEmailPlainTextBody() {
        return renderedEmail.getPlainTextBody();
    }
    
    public String getMergedEmailSubject() {
        return renderedEmail.getSubject();
    }
}

2 - DynamicEmail.vfc

<apex:component controller="DynamicEmailController" access="global">
    <apex:attribute name="RelatedToId" assignTo="{!emailRelatedToId}" type="String" description="Id of the RelatedTo Object for the email message"/>
    <apex:attribute name="TemplateId" assignTo="{!emailTemplateId}" type="String" description="The label that contains the template Id" />
    <apex:attribute name="HTML" assignTo="{!isHTML}" type="Boolean" description="Set to true if you want HTML" />
    <apex:outputText escape="false" value="{!mergedEmailHtmlBody}" rendered="{!isHtml}"/> 
    <apex:outputText escape="false" value="{!mergedEmailPlainTextBody}" rendered="{!!isHtml}"/>
</apex:component>

3 - DynamicAccountEmail.eml

<messaging:emailTemplate subject="{!SUBSTITUTE($Label.AccountEmailSubject,'{0}', relatedTo.Name)}" 
  relatedToType="Account" language="{!relatedTo.Language__c}">

    <messaging:htmlEmailBody >
        <c:DynamicEmail RelatedToId="{!relatedTo.Id}" TemplateId="{!$Label.AccountEmailId}" HTML="true"/>
    </messaging:htmlEmailBody>
    <messaging:plainTextEmailBody >
        <c:DynamicEmail RelatedToId="{!relatedTo.Id}" TemplateId="{!$Label.AccountEmailId}" HTML="false"/>
    </messaging:plainTextEmailBody>
</messaging:emailTemplate>