Solid Security's translation is handled in two different ways: one for the Basic version and another for the Pro version.
The Solid Security Basic plugin available on the wordpress.org directory is translated by volunteers on the official WordPress translation platform, which you can find here.
For translation guidance, you may contact the WordPress Slack#polyglots channel mentioned in the Before you get started section.
For translating the text strings that are only found within the Pro version, you can use a tool like Poedit or a translation plugin to generate the translation files (.po and .mo files).
To add your own translations, the files will need to go in their respective directories like the example path listed below.
/wp-content/languages/plugins/ithemes-security-pro/it-l10n-ithemes-security-pro-de_DE.mo
/wp-content/languages/plugins/ithemes-security-pro/it-l10n-ithemes-security-pro-de_DE.po