The styles selection is always in English (e.g. "Accent, Heading"), no matter what language is set for the user interface. I tried German and French and it seems that there is either a bug in the code not using the translations or that there are no translations - which would be quite a regression.
I confirmed this on both Linux and Windows.
This appears to be a regression from along the lines of
CommitDate: Wed Sep 28 11:42:56 2016 +0000
Remove old cell styles from calc
CommitDate: Sun Oct 2 13:51:26 2016 +0000
tdf#90937 Add a set of cell styles to calc
where we now have translatable strings not matching the internal style names anymore.
*** Bug 137226 has been marked as a duplicate of this bug. ***
Eike Rathke committed a patch related to this issue.
It has been pushed to "master":
Resolves: tdf#137370 Re-enable UI translated standard cell style names
It will be available in 7.1.0.
The patch should be included in the daily builds available at
https://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More
information about daily builds can be found at:
Affected users are encouraged to test the fix and report feedback.