datahub - default search lang is fre

[fvanderbiest]

According to https://github.com/georchestra/datadir/blob/7f82a8cf1f4fadce2d60cb6cd05bb1b02dc8fac7/datahub/conf/default.toml#L16 the datahub's search lang is "french" by default.

We should:

• comment this line, so that it searches on all available langs by default.
• also document it in https://github.com/georchestra/datadir/#switching-to-another-language---tasklist