Create a TMX file from two TMX files, that has the first file's source language and the second file's destination language.

rosetta.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Xml.Linq;

namespace ConsoleApp1
{
    internal class Program
    {
        static void Main(string[] args)
        {
            /* Create a TMX file from the source language of the first TMX file and the destination language
             * of the second TMX file, using an identical language (first's destination/second's source) as
             * the "Rosetta Stone".
             * 
             * You have:
             * * de-DE to en-US
             * * en-US to en-GB
             * 
             * You want:
             * * de-DE to en-GB
             */

            var first = LoadTranslations(@"C:\first.tmx", "de-DE", "en-US");
            var second = LoadTranslations(@"C:\second.tmx", "en-US", "en-GB");

            var merged = MergeTranslations(first, second);

            WriteTranslationsToTMX(merged, "C:\\merged.tmx", "de-DE", "en-GB");
        }

        static Dictionary<string, string> LoadTranslations(string filePath, string sourceLang, string targetLang)
        {
            XDocument doc = XDocument.Load(filePath);
            XNamespace ns = "http://www.w3.org/XML/1998/namespace"; // Default namespace for xml:lang
            var translations = new Dictionary<string, string>();

            foreach (var tu in doc.Descendants("tu"))
            {
                var sourceText = tu.Elements("tuv").FirstOrDefault(tuv => (string)tuv.Attribute(ns + "lang") == sourceLang)?.Element("seg")?.Value;
                var targetText = tu.Elements("tuv").FirstOrDefault(tuv => (string)tuv.Attribute(ns + "lang") == targetLang)?.Element("seg")?.Value;

                if (sourceText != null && targetText != null)
                    translations[sourceText] = targetText;
            }

            return translations;
        }

        static Dictionary<string, string> MergeTranslations(Dictionary<string, string> deUs, Dictionary<string, string> usUk)
        {
            var deUk = new Dictionary<string, string>();

            foreach (var deUsPair in deUs)
                if (deUsPair.Value != null && usUk.TryGetValue(deUsPair.Value, out var ukTranslation))
                    deUk[deUsPair.Key] = ukTranslation;

            return deUk;
        }

        static void WriteTranslationsToTMX(Dictionary<string, string> translations, string filePath, string firstLang, string secondLang)
        {
            XNamespace ns = "http://www.w3.org/XML/1998/namespace"; // Define the XML namespace
            var tmx = new XDocument(
                new XDeclaration("1.0", "UTF-8", "yes"),
                new XElement("tmx",
                    new XAttribute("version", "1.4"),
                    new XElement("header",
                        new XAttribute("creationtool", "TMXTranslationMerger"),
                        new XAttribute("creationtoolversion", "1.0"),
                        new XAttribute("datatype", "PlainText"),
                        new XAttribute("segtype", "sentence"),
                        new XAttribute("adminlang", "en"),
                        new XAttribute("srclang", "DE"),
                        new XAttribute("o-tmf", "ABC Translation Memory"),
                        new XAttribute("creationdate", DateTime.UtcNow.ToString("yyyyMMddTHHmmssZ")),
                        new XAttribute("creationid", "TMXMerger")
                    ),
                    new XElement("body",
                        translations.Select(kv =>
                            new XElement("tu",
                                new XElement("tuv",
                                    new XAttribute(XNamespace.Xml + "lang", firstLang),
                                    new XElement("seg", kv.Key)
                                ),
                                new XElement("tuv",
                                    new XAttribute(XNamespace.Xml + "lang", secondLang),
                                    new XElement("seg", kv.Value)
                                )
                            )
                        )
                    )
                )
            );

            tmx.Save(filePath);
        }
    }
}