SquirrelMail internationalization

This document should explain how SquirrelMail internationalization works and
provide information about some aspects of implementation.

1. Supported languages
2. $languages array
3. XTRA_CODE functions
4. Display of different charsets
5. IMAP folder names

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1. Supported languages
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Valid language codes are:
* ar    - Arabic, windows-1256 charset
* bg_BG - Bulgarian, windows-1251 charset
* ca_ES - Catalan, iso-8859-1 charset
* cs_CZ - Czech, iso-8859-2 charset
* cy_GB - Welsh, iso-8859-1 charset
* da_DK - Danish, iso-8859-1 charset
* de_DE - German, iso-8859-1 charset
* el_GR - Greek, iso-8859-7 charset
* en_GB - British, iso-8859-15 charset
* en_US - English, charset depends on $default_charset
* es_ES - Spanish, iso-8859-1 charset
* et_EE - Estonian, iso-8859-15 charset
* eu_ES - Basque, iso-8859-1 charset
* fa_IR - Farsi, utf-8 charset
* fi_FI - Finnish, iso-8859-1 charset
* fo_FO - Faroese, iso-8859-1 charset
* fr_FR - French, iso-8859-1 charset
* he_IL - Hebrew, windows-1255 charset
* hr_HR - Croatian, iso-8859-2 charset
* hu_HU - Hungarian, iso-8859-2 charset
* id_ID - Indonesian, iso-8859-1 charset
* is_IS - Icelandic, iso-8859-1 charset
* it_IT - Italian, iso-8859-1 charset
* ja_JP - Japanese, euc-jp charset (emails are created in iso-2022-jp)
* ko_KR - Korean, euc-kr charset
* lt_LT - Lithuanian, utf-8 charset
* ms_MY - Malay, iso-8859-1 charset
* nb_NO - Norwegian (Bokmal), iso-8859-1 charset
* nl_NL - Dutch, iso-8859-1 charset
* nn_NO - Norwegian (Nynorsk), iso-8859-1 charset
* pl_PL - Polish, iso-8859-2 charset
* pt_BR - Portuguese (Brazil), iso-8859-1 charset
* pt_PT - Portuguese (Portugal), iso-8859-1 charset
* ro_RO - Romanian, iso-8859-2 charset
* ru_UA - Ukrainian Russian, koi8-r charset
* ru_RU - Russian, utf-8 charset
* sk_SK - Slovak, iso-8859-2 charset
* sl_SI - Slovenian, iso-8859-2 charset
* sr_YU - Serbian, iso-8859-2 charset
* sv_SE - Swedish, iso-8859-1 charset
* ug    - Uighur, utf-8 charset (some systems don't support Uighur system locale)
* th_TH - Thai, tis-620 charset
* tl_PH - Tagalog, iso-8859-1 charset (main translation is missing, only some plugins are translated)
* tr_TR - Turkish, iso-8859-9 charset
* uk_UA - Ukrainian, koi8-u charset
* zh_CN - Chinese Simplified, gb2312 charset
* zh_TW - Chinese Traditional, big5 charset

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2. $languages array
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TODO: add explanation of $languages array keys

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3. XTRA_CODE functions
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XTRA_CODE functions provides way to change interface behaviour, when translation
requires special handling of some squirrelmail functions. Functions are enabled
by setting XTRA_CODE option in $languages array and providing appropriate
functions. First part of function name is word listed in
$languages['language_code']['XTRA_CODE'] setting. Second part is one of special
keywords. Possible values:
* _decode
Used in src/compose.php, src/i18n.php, src/view_text.php, functions/mime.php
Requires mbstring support

* _encode
Used in src/compose.php, src/read_body.php

* _encodeheader
Used in functions/mime.php
Returning function

* _decodeheader
Used in functions/mime.php
Returning function

* _downloadfilename
Used in functions/mime.php

* _utf7_imap_encode
Used in functions/imap_utf7_local.php
Returning function

* _utf7_imap_decode
Used in functions/imap_utf7_local.php
Returning function

* _strimwidth
Used in functions/mailbox_display.php
Returning function

* _wordwrap
Used in functions/strings.php (sqWordWrap)

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4. Display of different charsets
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TODO: add explanation of decoding functions

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5. IMAP folder names
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TODO: add explanation about UTF7-IMAP and mbstring