Save member's lang in a separated column

The idea would be to ease the front end where `currentMember.lang` would be easily feasible.

Currently: `currentMember.type === Individual && currentMember.extra.lang`
There are two problems: lang is saved in extra, which only exists in Members. We have to check the type to know if we can access extra.

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To go further: we want to remove the `extra` column. It currently contains:
```
type MemberExtra = {
hasAvatar?: boolean;
lang?: string;
emailFreq?: `${EmailFrequency}` | EmailFrequency;
hasCompletedTour?: boolean;
};
```
We can remove hasCompletedTour.
If we make a packed member with the avatar, we would not need `hasAvatar`. Or we keep it to prevent call during the pack logic.