Ok, I confirmed that with the patch from bug 1617669 the header is not translated neither on the code nor hot load. So my hypothesis was correct - we were applying the translation too early (in template), and it was localized using document's l10n. Instead, you should call `document.l10n.connectRoot` in `connectedCallback`.
Bug 1608197 Comment 11 Edit History
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Ok, I confirmed that with the patch from bug 1617669 the header is not translated neither on the cold nor hot load. So my hypothesis was correct - we were applying the translation too early (in template), and it was localized using document's l10n. Instead, you should call `document.l10n.connectRoot` in `connectedCallback`.