Defining a language
You can add a new language by going to Settings > Languages. Your existing languages are listed there.
At the bottom of the page are two links:
"Load predefined languages and sample stories"
This link leads to a list of over 50 languages predefined in Lute, with sample stories and dictionaries pre-configured. Click a language and it will be loaded so you can try it out.
If the language you're interested in is not present in the predefined languages, you can create a new language with the "Create new" link.
"Create new"
This link leads to a form for you to define your language.
If your language is similar to one of the predefined languages, you can use that as a "template" by selecting it from the dropdown at the top of the form and clicking "go."
If your language is new, you can check a spreadsheet created by the Lute community, which contains many languages and dictionaries. These are gradually being ported into Lute as volunteers find time.
Then add dictionaries, and you're pretty much set.