locale.translations

Several languages; lang picks one. Type a complete table as UIStrings (satisfies UIStrings) and the compiler lists every key you have not translated. Load it however you like (a dynamic import() per language works), but have it in hand before you build the viewer: translations are read once, when the viewer is built. A key the SDK does not have is ignored without a warning, so after upgrading, compare your table with EN_STRINGS.

import { EN_STRINGS } from '@kernstra/pdf-web-sdk';
const missing = Object.keys(EN_STRINGS).filter((key) => !(key in german));