Those are redundant, and yaml parser strips them in fact. By removing
them, loading and saving yaml file without any change indeed produce the
same output. This is useful for prepare_for_translation.py script (which
adds lang and ref tags) - to produce only change that indeed was made.
There appears to be no strong reason that these pages are using a
different layout, and having this separate layout significantly
increases the ongoing doc maintenance burden.
Those are fields used by the language switcher to correlate pages across
different languages, even if they have different names/paths/titles.
They are generated with the prepare_for_translation.py script.
