localization: default to sentence case for non-english titles

[anabelle2001]

Very few languages capitalize each word in a title:

https://linguistics.stackexchange.com/questions/32182/what-are-the-title-capitalization-rules-in-some-languages

https://ux.stackexchange.com/questions/72790/is-it-default-that-an-english-version-is-always-title-case-while-other-language/72813#72813

By default, de-arrow should use "Sentence case" rules for non-english titles.

[ajayyy]

This is already the case, but the language detection is not perfect

Also, if your browser's language is not english, the default is sentence case instead.