Ardour's user interface can be translated into many languages. For this the application uses gettext engine. It works like this: there is a plain text editable PO file for every language that contains a number of sequences like: a line in English, a line in target language and a reference to a line in source code where original text in English comes from. The build system then creates a binary version of each PO file with MO file extension and places it in a system directory where running instance of Ardour will pick and use it.
Ardour has two main PO files to translate: for user interface and for Ardour's library. Some messages from the library are actually passed to user interface (various warnings, for example), so ideally you have to translate both. For example, Russian translations are located here:
Every PO file's name is a two-character code, e.g. 'de' for German, 'es' for Spanish and so on. However it's fairly common to have localizations for local versions of a language, e.g. 'pt_BR' would stand for Brazilian Portuguese, and 'es_MX' would stand for Mexican Spanish. Please refer to ISO language code table for details.
Often when you go through translation hunting for untranslated messages, you stumble upon things you have no idea where they come from. To assist you there most advanced PO editors have means to tell you which source code file it comes from, or, in some cases, even show the context in source code. Ardour has pretty much meaningful names for source code files, so it should help. If it doesn't, please ask on IRC for help.
When Ardour runs, this "%1" will be substituted with "Ardour" so that the whole phrase will be "Cannot load XML for session from Ardour". If Mixbus (an commercial Ardour's spin-off) runs, this will be "Cannot load XML for session from Mixbus" instead. You get the idea. So just place this variable in your translation in a way that makes the phrase sound natural in your language and make sure you know where the phrase comes from (see above) so that you know how to translate it correctly.
English has two plural forms, but many other languages have three and even four plural forms. Some parts of Ardour's code respect that, but some not (check for existing bug reports). A PO editor usually simplifies translating every plural form by putting every plural form into a dedicated tab.
Not every PO editor can calculate the equation for the plural forms in your language, but you can look it up in headers of PO files from some application such a GIMP or Inkscape. Here is an example for Russian:
GTranslator. Usually considered as GNOME translation tool, but has very few actual GNOME dependencies. Feature-wise it's somewhere between poEdit and Lokalize.
