I have created a signal-based Business Process that fires on modifying the data in Opportunity.

To Read the Email Template I write the following code:

using System;
	using System.Collections.Generic;
	using System.Linq;
	using Newtonsoft.Json;
	using Terrasoft.Common;
	using Terrasoft.Configuration;
	using Terrasoft.Configuration.Utils;
	using Terrasoft.Core;
	using Terrasoft.Core.Configuration;
	using Terrasoft.Core.DB;
	using Terrasoft.Core.Entities;
	using Terrasoft.Core.OperationLog;
	using SystemSettings = Terrasoft.Core.Configuration.SysSettings;
 
 
	public class EmailTemplateManager
	{
 
		private UserConnection _userConnection;
		private MacrosHelperService _macrosService;
		private readonly Dictionary<Guid, Entity> _cachedTemplates = new Dictionary<Guid, Entity>();
 
		public EmailTemplateManager(UserConnection userConnection) : base() {
			_userConnection = userConnection;
			MacrosHelperV2 macrosHelper = new GlobalMacrosHelper();
			macrosHelper.UserConnection = userConnection;
			_macrosService = new MacrosHelperService(macrosHelper, userConnection);
		}
 
		public string GetEmailTemplateSchemaName() {
			return _userConnection.GetFeatureState("EmailMessageMultiLanguage") == 0
				? "EmailTemplate"
				: "EmailTemplateLang";
		}
 
		public string GetTemplateBody(Guid recordId, Guid templateId) {
			var template = GetTemplate(templateId);
			if(template != null) {
				var schemaName = GetSchemaNameFromId(template.GetTypedColumnValue<Guid>("ObjectId"));
				var body = this.GetTemplateBody(new MacrosHelperServiceRequest
				{
					EntityId = recordId,
					EntityName = schemaName,
					TemplateId = templateId
				});
				return body;
			}
			return string.Empty;
		}
 
		public string GetTemplateBody(MacrosHelperServiceRequest request) {
			MacrosHelperServiceResponse template = null;
			string body = string.Empty;
			if (_userConnection.GetIsFeatureEnabled("EmailMessageMultiLanguageV2")) {
				template = _macrosService.GetMultiLanguageTextTemplate(request);
				body = template.TextTemplate;
			} else {
				template = _macrosService.GetMultiLanguageTextTemplate(request);
				body = template.TextTemplate;
			}
			return body;
		}
 
		public string GetTemplateSubject(MacrosHelperServiceRequest request) {
			MacrosHelperServiceResponse template = _macrosService.GetMultiLanguageTextTemplate(request);
			string title = template.SubjectTemplate;
			return title;
		}
 
		private bool IsNeedChangeMacrosCulture() {
			return _userConnection.GetIsFeatureEnabled("UseMacrosAdditionalParameters")
				&& _userConnection.GetIsFeatureEnabled("EmailMessageMultiLanguageV2");
		}
 
		public string GetTemplateSubject(Guid recordId, Guid templateId) {
			var template = GetTemplate(templateId);
			if(template != null) {
				var schemaName = GetSchemaNameFromId(template.GetTypedColumnValue<Guid>("ObjectId"));
				return GetTemplateSubject(new MacrosHelperServiceRequest
				{
					EntityId = recordId,
					EntityName = schemaName,
					TemplateId = templateId
				});
			}
			return string.Empty;
		}
 
		private string GetSchemaNameFromId(Guid objectId) {
			if(objectId != Guid.Empty) {
				var selectQuery  = new Select(_userConnection)
								.Column("Name")
								.From("SysSchema")
								.Where("UId")
									.IsEqual(Column.Parameter(objectId)) as Select;
 
				return selectQuery.ExecuteScalar<string>();
			}
			return string.Empty;
		}
 
		public Entity GetTemplate(Guid id) {
			Entity template;
			if (_cachedTemplates.ContainsKey(id)) {
				template = _cachedTemplates[id];
			} else {
				var esq = new EntitySchemaQuery(_userConnection.EntitySchemaManager, GetEmailTemplateSchemaName());
				esq.AddColumn("Object");
				esq.AddColumn("Subject");
				esq.AddColumn("Body");
				esq.Filters.Add(esq.CreateFilterWithParameters(FilterComparisonType.Equal, "Id", id));
				template = esq.GetEntity(_userConnection, id);
				_cachedTemplates.Add(id, template);
			}
			return template;
		}	
	}

To call this class in the Business Process I used this code:

try{
 
	var userConnection = GetUserConnection();
	var emailManager = new EmailTemplateManager(userConnection);
	var templateId = Get<Guid>("TemplateId");
	var recordId = Get<Guid>("RecordId");
	var subject = emailManager.GetTemplateSubject(recordId, templateId);
	var body = emailManager.GetTemplateBody(recordId, templateId);
	Set<String>("Body", body);
	Set<String>("Subject", subject);
}
catch(Exception ex){
	Set<String>("Subject", ex.Message);
	Set<String>("Body", ex.StackTrace);
}
return true;

To get the UserConnection in the background I used this Code:

private UserConnection GetUserConnection(){
	return Get<UserConnection>("UserConnection");
}

Here is the error I get:

Error Message: Error creating an instance of the "Terrasoft.Configuration.ILanguageIterator" class 

Stack Trace:   at Terrasoft.Core.Factories.ClassFactory.GetInstance[T](Func`1 action)
  at Terrasoft.Configuration.MLangContentFactory.GetContentKit(String iteratorTagName, String storeTagName)
  at QTECX.Email.EmailTemplateManager.GetTemplateSubject(String schemaName, Guid recordId, Guid tplId)
  at QTECX.Email.EmailTemplateManager.GetTemplateSubject(Guid recordId, Guid templateId)
  at Terrasoft.Core.Process.QTXTestEmailProcessMethodsWrapper.ScriptTask1Execute(ProcessExecutingContext context)