Migrating away from modeltranslation

[KalobTaulien]

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Documentation on how to migrate away from wagtail-modeltranslation.

[zerolab]

@KalobTaulien do you have time to revisit this PR?

[benjaoming]

Thanks for starting this!

I'm not sure how I would benefit from the current state of the guide -- I would like to know how to a) migrate the actual translations so they're not just deleted and b) some guidance on how the 1:1 translation of modeltranslation and wagtail-localize compare.

[zerolab]

@benjaoming have you seen https://www.cnc.io/en/blog/wagtailtrans-to-wagtail-localize-migration ? While not directly modeltranslation, it should hopefully provide a good starting point on the process.