Get the index filename for a given language code. Mirrors the rule used by getIndexFilename() in news-indexes.ts.
getIndexFilename()
news-indexes.ts
Language code
index.html for English, index-<lang>.html for everything else
index.html
index-<lang>.html
Get the index filename for a given language code. Mirrors the rule used by
getIndexFilename()innews-indexes.ts.