Two aspects:
1. In order to translate your portlet to a new language, just follow the standard procedures and deliver your portlet with Language_xx.properties resources (provided your language code is xx). In order to do this, you configure your portlet to have a localization resource in portlet.xml like this:
<resource-bundle>content.Language</resource-bundle>
Refer to the
Developer's Guide for more information, the line above is copied from there.
2. If the language you want to use (let's keep "xx") is not yet known to Liferay, open your portal-ext.properties and define the languages you want to have available. You do this, as documented above. For more information, see this excerpt from the original portal.properties
#
# Specify the available locales. Messages corresponding to a specific
# language are specified in properties files with file names matching that
# of content/Language_*.properties. These values can also be overridden in
# properties files with file names matching that of
# content/Language-ext_*.properties. Use a comma to separate
# each entry.
#
# If there is more than one locale for the same language, the first one in
# the list will be used when a translation is requested in another locale of
# the same language but a translated value cannot be found. For example, if
# there are two locales such us pt_BR and pt_PT (in this order), any key not
# found in pt_PT will be looked for in pt_BR.
#
# Language codes follow the ISO 639-1 standard. Content for each locale must
# be UTF-8 encoded.
#
# See the following links, which specify language and country codes:
# http://www.loc.gov/standards/iso639-2/php/code_list.php
# http://userpage.chemie.fu-berlin.de/diverse/doc/ISO_3166.html
#
locales=ar_SA,eu_ES,bg_BG,ca_AD,ca_ES,zh_CN,zh_TW,hr_HR,cs_CZ,da_DK,nl_NL,nl_BE,en_US,en_GB,en_AU,et_EE,fi_FI,fr_FR,fr_CA,gl_ES,de_DE,el_GR,iw_IL,hi_IN,hu_HU,in_ID,it_IT,ja_JP,ko_KR,lo_LA,lt_LT,nb_NO,fa_IR,pl_PL,pt_BR,pt_PT,ro_RO,ru_RU,sr_RS,sr_RS_latin,sl_SI,sk_SK,es_ES,sv_SE,tr_TR,uk_UA,vi_VN
#
# Specify the locales that are in beta. Go to http://translate.liferay.com
# to participate in the translation efforts.
#
locales.beta=ar_SA,eu_ES,bg_BG,ca_AD,zh_TW,hr_HR,cs_CZ,da_DK,nl_NL,nl_BE,en_GB,en_AU,et_EE,gl_ES,el_GR,hi_IN,in_ID,it_IT,ko_KR,lo_LA,lt_LT,nb_NO,fa_IR,pl_PL,pt_PT,ro_RO,ru_RU,sr_RS,sr_RS_latin,sl_SI,sk_SK,sv_SE,tr_TR,uk_UA,vi_VN
#
# Specify the locales that are enabled by default.
#
locales.enabled=ca_ES,zh_CN,en_US,fi_FI,fr_FR,de_DE,iw_IL,hu_HU,ja_JP,pt_BR,es_ES
thus, you'll need to add
# note the new "xx" entry at the beginning
locales=xx,ar_SA,eu_ES,bg_BG,ca_AD,ca_ES,zh_CN,zh_TW,hr_HR,cs_CZ,da_DK,nl_NL,nl_BE,en_US,en_GB,en_AU,et_EE,fi_FI,fr_FR,fr_CA,gl_ES,de_DE,el_GR,iw_IL,hi_IN,hu_HU,in_ID,it_IT,ja_JP,ko_KR,lo_LA,lt_LT,nb_NO,fa_IR,pl_PL,pt_BR,pt_PT,ro_RO,ru_RU,sr_RS,sr_RS_latin,sl_SI,sk_SK,es_ES,sv_SE,tr_TR,uk_UA,vi_VN
# note the new "xx" entry at the beginning, add all the languages you want to have available by default
locales.enabled=xx,en_US
As always, when changing portal-ext.properties: Restart.
Deploy your portlet. Enjoy
