Currently we *manually* fetch the "pdfjs-additional-layers" string and update the DOM-element, which was needed since we want to avoid triggering a bunch of otherwise unnecessary translation when appending the entire layer-tree to the DOM.
By introducing a new helper method in the `L10n`-class we can avoid this, and instead use a "data-l10n-id" attribute on the element (as most other viewer code does nowadays).
Given the length of the l10n-strings we can slightly reduce verbosity, and thus overall code-size, by introducing a helper method for fetching l10n-data.
While testing this I stumbled upon an issue in the `L10n`-class, where an optional chaining operator was placed incorrectly since the underlying method always return an Array; see 48e2a62ed4/fluent-dom/src/localization.js (L38-L77)
- Ensure that localization works in the GENERIC viewer, even if the necessary locale files cannot be loaded.
This was the behaviour prior to the introduction of Fluent, and it seems worthwhile to keep that (especially since we already bundle the en-US strings anyway).
- Let the `GenericL10n`-implementation use the *bundled* en-US strings directly when no language is provided.
- Remove the `NullL10n`-implementation, and simply fallback to `GenericL10n`, to reduce the maintenance burden of viewer-components localization.
- Indirectly, given the previous point, stop exporting `NullL10n` in the viewer-components since it's now removed.
Note that it was never really intended to be used directly and only existed as a fallback.
*Please note:* This doesn't affect the Firefox PDF Viewer, thanks to the use of import maps.
This is consistent with the implementation used in the (now removed) webL10n-library, and by only using lowercase language-codes internally in the `L10n`-implementations we should avoid future issues e.g. when users manually set the `locale`-option (in the default viewer).
To prevent the *standalone* viewer-components from breaking, we need to ensure that the `NullL10n`-interface won't accidentally diverge from the actual `L10n`-implementations.
- For the generic viewer we use @fluent/dom and @fluent/bundle
- For the builtin pdf viewer in Firefox, we set a localization url
and then we rely on document.l10n which is a DOMLocalization object.
