The Monero command line tools can be translated in various languages.
In order to use the same translation workflow as the future GUI, they
use Qt Linguist translation files. However, to avoid the dependencies
on Qt this normally implies, they use a custom loader to read those
files at runtime. In order to update, or build translations files, you
do need to have Qt tools installed, however. For translating, you need
either the Qt Linguist GUI, or another tool that supports Qt ts files,
such as Transifex. To run, you do not need anything Qt.

To update ts files after changing source code:

  ./utils/translations/update-translations.sh

To add a new language, eg Spanish (ISO code es):

  cp translations/monero.ts translations/monero_es.ts

To edit translations for Spanish:

  linguist translations/monero_es.ts

To build translations after modiying them:

  ./utils/translations/build-translations.sh

To test a translation:

  LANG=es ./build/release/bin/monero-wallet-cli

To add new translatable sources in the source:

  Use the tr(string) function if possible. If the code is in a class,
  and this class doesn't already have a tr() static function, add one,
  which uses a context named after what lupdate uses for the context,
  usually the fully qualified class name (eg, cryptonote::simple_wallet).
  If you need to use tr in code that's not in a class, you can use the
  fully qualified version (eg, simple_wallet::tr) of the one matching
  the context you want.
  Use QT_TRANSLATE_NOOP(string) if you want to specify a context manually.

If you're getting messages of the form:

  Class 'cryptonote::simple_wallet' lacks Q_OBJECT macro

all is fine, we don't actually need that here.