> Both private and business use of the DeepL Translator (free) service are free of charge.
DeepL Translator (無料版) は個人用途でもビジネス用途でも無料で使えるよ。

> However; forwarding our services to third parties for payment is strictly prohibited.
でも当社のサービス使って対価は得るなよ。

「OSSの翻訳に使ってOK」とか書いてないのでアウト。