System requirements:
Database
- MySQL 5.5.3/MariaDB 5.5.20/Percona Server 5.5.8 or higher with PDO and an InnoDB-compatible primary storage engine,
- PostgreSQL 9.1.2 or higher with PDO,
- SQLite 3.6.8 or higher
As you haven't a Delete button on interface translation page, you can't do that from the translation interface. But you can do in a small custom module.
Example : Delete string ID 346, 'Home'
$lid = 346;