Multilingual Proposal entry

**Is your feature request related to a problem? Please describe.** In a fully bilingual country, we'd like to make Proposals bilingual, with the option to switch between languages as easily as in the rest of Decidim. **Describe the solution you'd like** To allow multi-lingual Proposal entry, with a language toggle (e.g. English Francais) available on the Proposal Entry form. **Describe alternatives you've considered** We can include multilingual text within a single proposal, but it makes viewing the titles awkward for the minority language.