Hi lukyeslu,
Our Theme is translation Ready so if you just want to translate the text on 404 page then you can simply do it by using multilingual plugin. But if you want to customize the 404 page with some other content then you really need to hire a developer to customize the code for you. You have to customize the 404 page from the Theme Directory and make sure you customize the Code by making the Child Theme so the you want loose the customized code then updating the Theme to a newer Version in future.
Thanks
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