locale.lang

A BCP-47 tag, e.g. 'de-AT'. It picks a table from translations and drives numbers and dates through Intl: zoom percentages, file sizes (the number; the unit is the common.bytes.* key, so French can say Mo), page counts, and dates in the signatures panel. Dates use the language's medium style ("Oct 7, 2026", "07.10.2026"); a signing time in the signatures panel shows the time the file records, not converted to the reader's time zone. Lists such as the Image tool's accepted types are joined the language's way ("PNG, JPEG, or GIF"). The viewer sets lang on its own element from this value, so screen readers pronounce the labels in that language; dialogs that open outside the viewer carry it too.

An invalid lang formats numbers and dates in English (a translation stored under exactly that string is still used). lang and the keys of translations are read in their canonical form (de-at becomes de-AT).