This would be a very useful feature for multilingual users. Many people write in a language different from their OS, so configurable spellchecker settings make a real difference. Language-based systems need flexibility, just like tools that analyze structured data such as codice fiscale inverso, where accuracy depends on correct language rules. A customizable spellchecker would greatly improve the desktop app experience.

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Every writing Desktop Software (take LibreOffice) has the possibility to choose if the spell checker is activated or not and in which language the check takes place without depending on the language OS that is configured.

Many people know and write in multiple languages. Proofing language should not be tied to application language. I want to configure multiple language I commonly write in and spellchecker automatically picks the one I write in, or allow me to quickly switch between my languages from a drop down. Desktop app is most important to me, but mobile apps should also have this.

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I use English interfaces on the web and on my desktop, yet I write and read in multiple languages. Not being able to spellcheck in the email app is a drag and slows down my productivity.

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how is it that you're launching new services but the mail client app doesn't support multiple languages ??

please focus on the basics before launching anything else

Still an issue on current desktop versions (Windows 11). No OS-level or in-app workaround exists.

I have to write emails in Spanish and English and the lack of this functionality is a real pain.

At least Isaac's workaround worked.

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As a workaround -

1) Make sure Proton Mail are closed.
2) Open the following file.config/Proton Mail/Partitions/app/Preferences (very important it is this exact one!)
3) Change the dictionaries property to the language/locale which you want to have spelling for. E.g. here I have selected US English and Spanis; in Spain. :{"partition":{"per_host_zoom_levels":{"4966268783509498187":{}}},"spellcheck":{"dictionaries":["es-ES","en-US"],"dictionary":""}}
4) Save the file.
5) Open Proton Mail again.

This is a really big deal breaker and I hope they implement this soon.

I asked for the same feature when the beta version of Proton Mail desktop app was released and it's still not implemented.
That's the only reason why i can't use it and the first time I'm disappointed by the lack of advancement from Proton Team.
This is basic to implement and critical for most users in the world since we need to write in at least 2 languages nowadays.

“This would be a really helpful feature. The desktop app should definitely allow users to set a custom spell-checker language, especially for those who write in multiple languages. I often cover similar usability issues on my website, and small options like this can make a big difference in productivity and user comfort. for more info visit here: https://mincraftapk.com/

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I completely agree — having the ability to manually configure the spell-checker language in the Proton Mail desktop app would be extremely useful. Many users write emails in multiple languages, and relying only on the OS language creates unnecessary limitations. A built-in language selector (similar to what browsers use) would make the app much more flexible and user-friendly.

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...et Proton basé à Genève est incapable e communiquer dans une de nos trois langues nationales

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Email is primarily a text-based communication tool, so clarity and accuracy in writing are essential. Security means little if the message itself is poorly communicated.

Yes please! My system is set to English but most of my mails I type in my native language and everything has an annoying red underline.