Add recommended locales to the testing document (#1187)

* Add recommended locales to the testing document

* Apply Ezio's review suggestion

Co-authored-by: Ezio Melotti <ezio.melotti@gmail.com>

* Add reference to locale command for linux/mac

---------

Co-authored-by: Ezio Melotti <ezio.melotti@gmail.com>

testing/run-write-tests.rst

@@ -96,6 +96,25 @@ CPython checkout. The script tries to balance speed with thoroughness. But if
 you want the most thorough tests you should use the strenuous approach shown
 above.
 
+Locale support
+--------------
+
+Some tests require specific locales to run successfully. These locales are
+often non-default, non-English, non-UTF-8 locales. If a necessary locale is
+unavailable, the test is skipped or runs in the dry-run mode.
+Additional locales that you may find helpful to set up on developer's machines
+or buildbots include:
+
+* ``en_US`` (``en_US.utf8``, ``en_US.iso88591``) --- the standard default
+* ``de_DE`` (``de_DE.UTF-8``) or ``fr_FR`` (``fr_FR.utf8``, ``fr_FR.iso88591``,
+  ``fr_FR.iso885915@euro``) --- common non-English locales
+* ``tr_TR`` (``tr_TR.iso88599``) --- Turkish has different rules for upper/lower
+  cases of "i" and "I".
+* ``ps_AF`` --- used in ``test_decimal``
+
+On Linux and macOS, the ``locale`` command can be used to list available
+locales and change the settings. Environment variables ``LANG`` and those
+prefixed with ``LC_`` can be used to set the locale.
 
 Unexpected skips
 ----------------