Please follow the example and table below, to change the language of FTP Attachments® pop up window (For Attach, View, Update, Delete). User has to append &lang=Code (Refer Code from the below table) in the Custom Link URL manually as shown below.
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​for e.g:

Custom Link:

/apex/ENZ__LightningEval?URLToBeRedirect=https://app.ftpattachments.com/sforce/ftp/Attach.aspx&FieldName=ENZ__Opportunity__c&FieldValue={!Opportunity.Id}&OrgID={!Organization.Id}&SFUrl={!API.Partner_Server_URL_90}&NoWarnings=false&Folder=Opportunities/{!Account.Name}/{!Opportunity.Id}&lang=it

For Standalone user, replace {Yourdomain} with users domain name.

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