Write rather than Translate
I said, Translation is not easy, but it can be a good alternative .
However, as @nukeador has already mentioned in Post #2 on Problems finding public domain sentences, it would be more efficient and the quality of the text would be more reliable if you create your own text rather than translate it.
Translation requires an understanding of the foreign language and the ability to edit the words properly.
For example, you can use a machine to do automatic translation and then rework the generated text into something completely different by yourself. To the point of using it as a "material", I think anyone can make a corpus of foreign languages useful.
You may not be comfortable with the idea of writing it yourself.
But, as I mentioned in Ideas for finding public domain text, it can be done by tweet, chat or email.
If you don't have that either, then it can be a description of an everyday action you're doing, a landscape. Like, "My neighbor's dog is annoying," or "I posted this on the forum but no one is liking it". Your soliloquy will help everyone.
I like those sentences, they're easy and many people will enjoy reading them.
Everyone has the secret to creating a corpus.
青空文庫
- 公開中 作家リスト：全て - 青空文庫 (List of authors of available works)
- 青空文庫編 青空文庫収録ファイルの取り扱い規準 (Handling Standards for Aozora Bunko Files)
In Japanese
青空文庫には、著作権切れの作品を電子化したコンテンツがあります。
青空文庫から返信を頂きましたが、「取り扱い規準」のメタ情報やクレジットの希望は、あくまで「希望・期待」である、とのことでした。
著作権の発生する規定が「思想又は感情を創作的に表現したもの」で、著作物に「創作性」が必要なことを考慮すると、青空文庫内の著作権切れ作品は、パブリックドメインのままである（＝青空文庫は著作者ではない）と私は判断しました。
収集の際には、作品がパブリックドメインであるか確認することを忘れないで下さい（青空文庫には、著作権が存続する作品もあります。註記もありますが、収集する人が必ず自分で確認して下さい）。
著作権の解釈に関して誤りがあれば、ご指摘下さい。
註：旧字（正字）・旧仮名（歴史的仮名遣、正仮名遣）は、今日日常的には用いられていないので、リストから「新字新仮名」版を探すか、収集する人が新字・新仮名（現代仮名遣）に編集して下さい。
参考 (Japanese Copyright Law)：著作権法 - e-Gov法令検索
In English
Aozora Bunko contains digital versions of out-of-copyright works.
I received a reply from Aozora Bunko, stating that the metadata and credit wishes in the "取り扱い規準 (Handling Standards)" are just "hopes and expectations".
Considering the fact that the provision that creates copyright is "思想又は感情を創作的に表現したもの (creative expression of thought or feeling)" and that works need to be "創作性 (creative)", I decided that the out-of-copyright works in Aozora Bunko remain in the public domain (i.e. Aozora Bunko is not the author).
When collecting, don't forget to check whether the work is in the public domain (some works in the Aozora Bunko are still in the copyright. There are notes, but collectors should always check for themselves).
Please point out any errors in the interpretation of copyright.
Note: "旧字旧仮名" (Kyūjitai and Historical kana orthography) is not commonly used in Japan today, so please look for the "新字新仮名" version in the list, or edit it to Shinjitai and Modern kana usage by the collector.