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> The EU has had since 2009 to clarify this

They did, back in 2012. See point 3.6 here: https://ec.europa.eu/justice/article-29/documentation/opinio...

More recently, see point 49 here (in French): https://www.cnil.fr/sites/cnil/files/atoms/files/lignes_dire...

[ggreer]

Section 3.6 specifically says that you need a cookie consent notice if you save preferred language for more than one session (a few hours).

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It says that simply telling the user that the language setting uses a cookie is enough to obtain consent in this case. Not that you need a full blown banner. The CNIL even says you don't need consent to do it.