Use context language to select the right values for node/term forms.

There's already this concept of 'context language' in i18n, though not really used yet. We should:
- Implement it for taxonomy term pages, adding i18n_taxonomy_context_language()
- Use it for taxonomy and node forms to select the right value for menu items or attached terms.

See:
i18n_language_context() (recently renamed)
hook_i18n_context_language()

See also #1531002: Use proper content/interface/context language everywhere.