[SPI] Change ContentHandler->load \$translations to be array

It is documented and used as an array if not null, so make this enfoced
so we don't need to check all over.
Implement EZP-23134: Cache content load calls with translations

Since Stash is hierarchical, add a extra layer for translation string.
Merge pull request #921 from ezsystems/spi_cache_content_load_w_translations

EZP-23134: Cache content load calls with translations