date: align Thai locale formatting with GNU

[compilersutra]

Match GNU Thai solar calendar behavior for th_TH:

• %Y / %m / %d use Buddhist-era values
• %EY / %EC / %Ey use พ.ศ. era forms
• Thai month/weekday names match GNU

Fixes the empty/closed state of #12921 after a bad rebase.

[sylvestre]

sorry but what was not working before this PR ?

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/tail/pipe-f2. tests/tail/pipe-f2 is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/tail/pipe-f is now passing!