Comment 1 for bug 1755918

Actually, the root cause is something which Mozilla seems to have forgotten implementing in Firefox 59:

Starting with Firefox 59, the parameter which specifies your localized UI language setting is no longer general.useragent.locale, type string, but
intl.locale.requested, type string

Mozilla seems to have forgotten to implement an automatism which rewrites the pre-Firefox parameter
- general.useragent.locale=de # German as an example
+ intl.locale.requested=de

So the solution is adding intl.locale.requested manually in about:config and restarting Firefox 59.0.

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This bug report can be closed therefore.