The site's language
Omeka Classic has one language per installation, set by locale.name in application/config/config.ini, such as locale.name = "fr" or locale.name = "pt_BR". Left empty, it is English.
The theme takes the page language from it, so browsers, screen readers and search engines know which language the page is in. Dates written the machine way (1993-04-09) are shown as people write them in that language.
Right-to-left languages
For right-to-left languages such as Arabic, Hebrew, Persian and Urdu, the theme also sets the page direction: headers, menus, cards, sidebars and arrows all mirror, because the theme's styles use logical directions (start and end) throughout.
Translation
Omeka translates its own interface and its plugins' words into the site's language. The words the theme shares with them, such as "Items", "Collections", "Exhibits", "Search" or "Tags", appear in your language wherever Omeka already translates them. The theme's own words ("Cite this", "Rights & reuse", "Search the collection", the names and help texts on its settings page…) are in English: the theme does not include translations of its own yet.
Text you write yourself is shown as you write it, in any language: the theme's text settings (banner, welcome text, announcement, footer, header button), Simple Pages, exhibits and your items' metadata.
Multilingual sites
Since an Omeka Classic installation has one language, a common way to offer a collection in several languages is to write its pages in each language (a Simple Page per language, linked from the menu), or to run one installation per language.