Basic functionality used by almost every other module including comphelper, so even don't use that string helpers in this code to not create circular dependencies. Stick with sal and rtl!
If Microsoft introduced a new LCID for a locale that we previously defined as LANGUAGE_USER_..., for example LANGUAGE_CATALAN_VALENCIAN that we had as LANGUAGE_USER_CATALAN_VALENCIAN:
* include/i18nlangtag/lang.h
** add the new LANGUAGE_... value as defined by MS, here LANGUAGE_CATALAN_VALENCIAN
** rename the LANGUAGE_USER_... definition to LANGUAGE_OBSOLETE_USER_..., here LANGUAGE_USER_CATALAN_VALENCIAN to LANGUAGE_OBSOLETE_USER_CATALAN_VALENCIAN
** add a #define LANGUAGE_USER_CATALAN_VALENCIAN LANGUAGE_CATALAN_VALENCIAN
*** so svtools/source/misc/langtab.src (where the defined name is an identifier) and other places using LANGUAGE_USER_CATALAN_VALENCIAN do not need to be changed
* i18nlangtag/source/isolang/isolang.cxx
** insert a mapping with LANGUAGE_CATALAN_VALENCIAN before (!) the existing LANGUAGE_USER_CATALAN_VALENCIAN
** rename the LANGUAGE_USER_CATALAN_VALENCIAN to LANGUAGE_OBSOLETE_USER_CATALAN_VALENCIAN
*** so converting the tag maps to the new LANGUAGE_CATALAN_VALENCIAN and converting the old LANGUAGE_OBSOLETE_USER_CATALAN_VALENCIAN still maps to the tag.
* i18nlangtag/source/isolang/mslangid.cxx
** add an entry to MsLangId::getReplacementForObsoleteLanguage() to convert LANGUAGE_OBSOLETE_USER_CATALAN_VALENCIAN to LANGUAGE_CATALAN_VALENCIAN
When changing a (translation's) language tag (for example, 'ca-XV' to 'ca-valencia' or 'sh' to 'sr-Latn'):
* solenv/inc/langlist.mk
** replace the tag and sort alphabetically
* in translations/source do git mv old-tag new-tag
** note that translations is a git submodule so https://wiki.documentfoundation.org/Development/Submodules applies
* i18nlangtag/source/isolang/isolang.cxx
** maintain the old tag's mapping entry in aImplIsoLangEntries to be able to read existing documents using that code
** add the new tag's mapping to aImplBcp47CountryEntries or aImplIsoLangScriptEntries
** change mbOverrideExists from false to true in aImplIsoLangScriptEntries
* i18nlangtag/source/languagetag/languagetag.cxx
** add the new tag's fallback strings to the fallback of the old tag in LanguageTag::getFallbackStrings()
* i18nlangtag/qa/cppunit/test_languagetag.cxx
** add a unit test for the new tag and old tag
* instsetoo_native/util/pack.lst
** replace the tag in List_of_languages and sort alphabetically
* i18npool/source/localedata/data/*.xml for example i18npool/source/localedata/data/sh_RS.xml
** in the <LC_INFO> element
*** change <LangID> to 'qlt'
*** after the <Country> element add a <Variant> element with the new full BCP 47 tag, for example 'sr-Latn-RS'
**** note that <Variant> has no <VariantID> or <DefaultName> child elements
** if any of the other *.xml files reference the locale in a ref="..." attribute, change those too; note that these references use '_' underscore instead of '-' hyphen just like the file names do
** rename sh_RS.xml to sr_Latn_RS.xml, git mv sh_RS.xml sr_Latn_RS.xml
* i18npool/source/localedata/localedata.cxx
** in aLibTable change the entry from old "sh_RS" to new "sr_Latn_RS", do not sort the table
* i18npool/Library_localedata_*.mk for example i18npool/Library_localedata_euro.mk
** change the entry for the changed .xml file, for example CustomTarget/i18npool/localedata/localedata_sh_RS to CustomTarget/i18npool/localedata/localedata_sr_Latn_RS, sort the list alphabetically
* dictionaries/*/dictionaries.xcu for example dictionaries/sr/dictionaries.xcu
** change the affected <node oor:name="..."> elements to something corresponding, for example <node oor:name="HunSpellDic_sh" ...> to <node oor:name="HunSpellDic_sr_Latn" ...>
** in the "Locales" properties change the <value> element, for example <value>sh-RS</value> to <value>sr-Latn-RS</value>
