Interesting bug.

1. I created a pivot table with UI language nl-Dutch active.
   Styles in the pivot table, are the localized ones.
   E.g. "Draaitabel categorie".
   ( I changed the cell background of some of the styles )

2. Open this file in LibreOffice with (Tools>Options>Language>) UI in en-US 

3. Change one of the filters.
  > The pivot table is adapted, but.. the background colors disappear
  > It turns out that now the en-US named styles are used in the Pivot table
   E.g. "Pivot Table Category"

Will attach a test file and a screenshots before&after

Seen in master and in Version: 5.4.1.2
Build ID: ea7cb86e6eeb2bf3a5af73a8f7777ac570321527
CPU threads: 4; OS: Linux 4.13; UI render: default; VCL: gtk2; 
Locale: nl-NL (nl_NL.UTF-8); Calc: group