There’re 2 methods:

Method 1: Use a plugin such as Loco Translate (or any translation plugin) and follow its instruction.

Method 2: Manually translate your theme:

Rome includes rome.pot file in rome/languages/ foler (you’ll find it once unzip theme file rome.zip). Open it with some Translation software, say POEdit. Translate all words you need and save 2 files *po and *mo as suitable names. For example, if you want to translate to Germany, save your files as de_DE.po and de_DE.mo. See Installing WordPress In Your Language for more details about file name.

Open file wp-config.php in your WordPress root folder and looking for the line define(‘WPLANG’, ”); Replace ” by the language you want. See Installing WordPress in Your Language for more details.