Translation

Blank includes a .pot file in wp-content/themes/blank/languages/. Open it with translation software such as Poedit, translate the strings you need, and save the .po and .mo files with appropriate names. For example, to translate into German, save your files as de_DE.po and de_DE.mo. See WordPress in Your Language for more about file naming.

On modern WordPress you can simply set your language under Settings » General » Site Language. (On older versions, you edited the WPLANG definition in wp-config.php.)

Example: translating into French

Set your site language to French under Settings » General » Site Language. (On older WordPress, open wp-config.php and change define('WPLANG', ''); to define('WPLANG', 'fr_FR'); — for German it would be define('WPLANG', 'de_DE');.)
In your theme, rename default.po and default.mo to fr_FR.po and fr_FR.mo.
Open fr_FR.po with Poedit and start editing. Save, and see the translation in action.