[fix] qwant - API error::locale must be one ..

The request function should not request a language (aka locale) that is not
supported by qwant. Select a locale like zh-TW ends in qwant's API error:

  ERROR searx.engines.qwant news: exception : \
  API error::locale must be one of the following values: \
    en_gb, en_ie, en_us, en_ca, en_my, en_au, en_nz, de_de, de_ch, de_at, fr_fr, \
    fr_be, fr_ch, fr_ca, fr_ad, fc_ca, co_fr, es_es, es_ar, es_cl, es_co, es_mx, \
    es_pe, es_ad, ca_es, ca_ad, ca_fr, eu_es, eu_fr, it_it, it_ch, pt_pt, pt_ad, \
    nl_be, nl_nl

The existing searx.utils.match_language function is unsuitable for this purpose,
it is replaced by function searx.locales.get_engine_locale that is based on the
methods from the babel package.

The quant's _fetch_supported_languages function has been revised to filter out
languages 8aka locales) not supported by qwant.

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>