Fixes language translation in model header field

Translation in a model header field was broken due to a comparison between string 'true' and boolean true.

This change will also make the model type accept 'yes', '1', 'on' for the translate parameters.
Based on the parent FOFFormHeaderFieldselectable
Merge pull request #265 from Radio-Actif/fix/language-header-model

Fixes language translation in model header field