Check child theme language folder first when defining $lang

DMS should check it there's a language folder:

1. Firstly in the child theme directory (<strong>not the case before this commit</strong>),
2. Secondly in the customize plugin directory,
3. before defaulting to main DMS directory language folder.

Should fix [bug #321: "Translating DMS: "child theme method" doesn't work"](https://github.com/pagelines/DMS/issues/321/ "Bug #321").