Make language-select HTML usable
Be explicit about types
Style #flash better. Needs an HTML change.
Rewrite fix for #554.

* Don't use z-index: that makes upload-progress dialog invisible.
* Set heights of hidden elements to 0: that makes scrollbar right height
Allow setting language on CSV import. Fixes #545. Fixes #546.

This particular commit should be reverted, as uses a totally
inflexible GET query string parameter in CSV import.