Android/Facebook missing default translation 'warning' errors

From nowhere, my Xamarin Forms solution has generated 22 errors all in an Android project, all complaining that lots of 'com_facebook_XXXX has no default translation'. I'm using a fully up to date version of VS2017 (15.4.2).
I have no idea what has caused this and googling the individual errors doesnt seems to turn anything relevant up.

I can see in Resource.Designer.cs that these constants are defined:

public const int com_facebook_smart_login_confirmation_title_m2gender = 2131230763;

// aapt resource value: 0x7f080016
public const int com_facebook_tooltip_default = 2131230742;

// aapt resource value: 0x7f08002c
public const int com_facebook_tooltip_default_f1gender = 2131230764;

// aapt resource value: 0x7f08002d
public const int com_facebook_tooltip_default_m2gender = 2131230765;

But what do I do with them?

Has anyone seen this before or know what has caused it?

Thanks