apps: Support empty lang attribute

[Venefilyn]

When we look for a specific language for an element in AppStream, such
as Summary or Description, we first try to find the requested language
otherwise we fallback to the first element we found.

As the first item might not be the default language in some cases we
could get a format like this

<summary xml:lang="fr">Gestionnaire d'abonnement dans Cockpit</summary>
<summary>Subscription Manager in Cockpit</summary>
<summary xml:lang="it">Gestore abbonamenti nel Cockpit</summary>
<summary xml:lang="ja">Cockpit のサブスクリプションマネージャー</summary>
<summary xml:lang="ka">გამოწერების მმართველი Cockpit-ში</summary>
<summary xml:lang="ko">Cockpit 서브스크립션 관리자</summary>
<summary xml:lang="ru">Менеджер подписок в Cockpit</summary>
<summary xml:lang="zh-Hans-CN">Cockpit 中的订阅管理器</summary>

In this case we'd choose the first summary found which would be French
instead of English.

Instead, we can first look for the language we want, and if we found an
element without language attribute we use that as a fallback. If both
fail we still fallback to the first element found.

Signed-off-by: Freya Gustavsson freya@venefilyn.se

[allisonkarlitskaya]

I feel like the "choose the element with no language tag" should also apply to the case that lang is None? ie: we probably want to loop in that case as well now, and select the "no language" item and return it.

A comment at the bottom "in case we don't find the requested language or an element without a language tag, we use the first tag as a final fallback" might be useful.

It might also be useful to add typing here, in general because it's nice, but specifically since it would force us to reason about the possibility that xml.find() returns None...

[allisonkarlitskaya]

... also, those test fails seem possibly relevant.