LinguaBook · Last updated: August 2026
LinguaBook is a book reader. Most of what it does happens entirely on your device, and you can use it without an account. Two optional features do send data to our servers: creating an account, and backing up your library. This policy sets out exactly what those involve.
Your imported books, your reading progress, and your settings (language, font, theme, tutorial state) are stored on your device. Unless you turn on Backup and sync, none of it reaches us.
LinguaBook never searches your device's storage. When you import a book or a font, Android's own file picker opens and you choose the file yourself; the app receives that file and nothing else. It has no permission to read your storage, and none to ask for.
LinguaBook can be used without an account. One is required only for Backup and sync, which is part of the Premium features.
Sign-in is handled by Firebase Authentication using Google Sign-In or Sign in with Apple. We never see or receive your password. From the sign-in provider we receive:
The email address is used to identify your account and to tell you which account a purchase belongs to. The profile picture is displayed in the app so you can see which account is connected; it is loaded directly from the provider and is not copied to our servers.
When you enable this feature, the following is stored on our servers:
This data is tied to your account and accessible to you alone. Access is enforced on the server side, not merely in the app. Each account may store up to 500 MB.
You can remove all of it at any time from the app: Account → Delete backup removes the data and keeps your account; Account → Delete my account removes both.
When you buy Premium, the app store issues a purchase receipt. The app sends that receipt to our server, which checks it directly with Google Play and records the purchase token, the product identifier, your account identifier, and the expiry date. This is what lets us grant access to Backup and sync, and what prevents a single purchase from being used across several accounts.
All payments are processed by the app store (Google Play or the Apple App Store). We never receive or store your payment details.
When you select a word or sentence to translate, that text is sent to Google's translation service to obtain the translation. No data identifying you is attached to the request. If you download an offline language, translation then happens entirely on your device and nothing is sent anywhere.
Requests to our servers are checked with Firebase App Check (Play Integrity on Android, App Attest on iOS). This confirms that a request comes from a genuine copy of the app rather than from a script, and protects the service from abuse. It is handled by Google and involves no personal data about you.
If you contact us through the in-app feedback form, the content of your message is transmitted through the Formspree service so that it reaches us. Please do not include sensitive personal information in your message.
The app shows no advertising and uses no analytics or tracking tools. We do not build profiles, and we do not sell or share your data with anyone for advertising purposes.
Account data and backups are held on Google Cloud infrastructure operated through Firebase, on servers located in the United States. The processors we rely on are Google (Firebase, Google Play, translation) and Formspree (feedback messages only).
Your backup is kept for as long as your account exists. Deleting it from the app removes it from our servers.
Records of a purchase are kept for as long as needed to honour that purchase and to meet accounting obligations, even after an account is deleted.
Deleting your account does not cancel your subscription. A subscription is managed by the app store and must be cancelled there.
You may access, correct, export or delete your data. Deletion and removal of your backup are available directly in the app, without having to ask us. For anything else, contact us using the details below and we will respond within one month.
The app is not directed at children and does not knowingly collect any data about them.
This policy may be updated from time to time. Any significant change will be reflected on this page with an updated date.
If you have any questions about this policy, you can reach us through the contact form available in the LinguaBook app (Settings → Send feedback).