Update git submodules
- Update extensions/MultimediaViewer from branch 'REL1_39' to 21c55e18103e1afb25ddfbb782bff8e125e5b4fd
- Localisation updates from https://translatewiki.net.
Change-Id: I0baee8aac189d597e9731e665a4be2e219bce289
- Localisation updates from https://translatewiki.net.