Hi @Jérôme Lucas ,
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Based on my test in Chrome, after I logged in to outlook.office.com, I can install the Outlook PWA successfully. Which install button do you mean? Is it in the below figure?
In order to further confirm if the issue is related to Chrome, it's suggested that you could try in other browser(such as Edge) to check if you can install PWA successfully.
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Well that worked. Thank you. I definitely had it installed previously when language was set to English (United Kingdom). What a surprise!
Just tried it and that worked for me too :)