In LSP.expert, you can create documents in any language you want (see this article: https://help.lsp.expert/en/articles/722489-can-i-translate-my-documents-in-another-language), but you can also create bilingual invoices.
For that, you have to create a new document configuration under Settings > Document and email configuration. Click on 'Add a new document and email configuration'.
Once you're in this new document configuration, go to the Translations tab and translate all the fields in the two languages of your choice, as shown in the screenshot below:
When it's done, click on Save.
In order to have this bilingual document configuration applied to the documents you create for your clients, don't forget to go and select this document configuration in the details of the clients concerned, under Clients. For this, you have to edit the Overview panel of the client.
And that's it. All the invoices you'll create for this/these clients will be bilingual, based on the new document configuration you just created.