Support overriding of i18n files from within a theme

[bbranan]

Is your feature request related to a problem? Please describe.

When applying customizations to the DSpace 7 UI, the majority of all changes can be done from within a theme. This allows for keeping customizations for a site separate from the base/default codebase, which is valuable for several reasons. Unfortunately, it is not currently possible to maintain this separation for UI label text that resides in the src/assets/i18n directory. These translation files must be edited in place, which is a departure from the theme model.

Describe the solution you'd like

The ideal solution would be to allow creating files in src/themes/{my-theme}/assets/i18n that are used to override specific i18n label values. The file names could be required to match between the two i18n directories, to support updating any language. Only the labels to be updated should be needed in the theme, not a full copy of the translation file.

Describe alternatives or workarounds you've considered

The immediate workaround is to update translation files directly within the top-level src/assets/i18n directory.

[mwoodiupui]

Let me add weight to the desire to override specific catalog entries, not just copy the whole catalog and hack it. We have done something similar with XMLUI and I find it much easier to work with just our changed texts.