As I can see, all language strings have been loaded with their values in back-end
No they havent:
Login with your the credentials you have and set your login language to nl-BE
Below is a direct link to the addons page
/administrator/index.php?option=com_tz_portfolio_plus&view=addons
Take a look at the 2 that are activated:
1 is properly loading the langauge file and is using the string "Media type - Rooster Gallerij", with language debug enabled it shows as **Media type - Rooster gallerij** (the ** means it loaded a language value)
the other is using the string value "plg_mediatype_flipbook_gallery", with language debug enabled, it STAYS "plg_mediatype_flipbook_gallery"
Now check the loaded language files
**Geladen** : JROOT/components/com_tz_portfolio_plus/addons/mediatype/grid_gallery/language/nl-BE/nl-BE.tp_addon_mediatype_grid_gallery.ini <== loaded
**Niet geladen** : JROOT/components/com_tz_portfolio_plus/addons/mediatype/flipbook_gallery/language/nl-BE/nl-BE.tp_addon_mediatype_flipbook_gallery.ini <== NOT LOADED, despite being in the proper directory
To translate the word "Page", you can go to Admin > Extensions > Language Override
I shouldn't have to use a language override, since that string is defined in the language file. The file just isn't being loaded, that's the problem
As I can see, all language strings of addons in backend have already been translated.
Here's a link to the currently only active article
/administrator/index.php?option=com_tz_portfolio_plus&view=article&layout=edit&id=2
Open the section "Artikel weergave opties" and scroll all the way down.
Notice how **ROOSTER GALLERIJ OPTIES** has properly loaded all the language strings, but ALL the ones for flipbook are listed as ??PLG_MEDIATYPE_FLIPBOOK_GALLERY_OPTIONS_LABEL?? <== meaning, unknown string, language string not loaded.
Same thing under "Media type" on the left, ??flipbook_gallery??
the language file for flipbook gallery is NOT being loaded, joomla sais so via language debug