message: Recognize X-Loco-Source-Locale
  Serves the same purpose as X-Source-Language, so fall back to it if
  present.
diff: @@ -983,6 +983,8 @@ class LoadParser : public CatalogParser
  Language GetSpecifiedMsgidLanguage()
  {
  auto x_srclang = m_catalog.m_header.GetHeader("X-Source-Language");
  + if (x_srclang.empty())
  + x_srclang = m_catalog.m_header.GetHeader("X-Loco-Source-Locale");
  if (!x_srclang.empty())
  {
  auto parsed = Language::TryParse(str::to_utf8(x_srclang));
  