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Our initial targets should be former victims of British colonialism:
en.wikipedia.org/wiki/List_of_countries_and_territories_with_the_Union_Jack_displayed_on_their_flag
Good designs for alternative national flags in fact have already been proposed in some cases:
en.wikipedia.org/wiki/List_of_proposed_Australian_flags
en.wikipedia.org/wiki/List_of_proposed_New_Zealand_flags
en.wikipedia.org/wiki/Flag_of_Tuvalu#Changes_and_controversies
therefore in these cases it is mainly a matter of pushing for replacement of the existing flags. Of course those designs which continue to include the colonizer flag are unacceptable, as are designs which include tasteless items such as boomerangs (a hunting weapon).
Elsewhere, alternative flags have yet to be designed:
en.wikipedia.org/wiki/Flag_of_Fiji#Proposal_for_a_new_flag
And in the remaining cases political independence has not yet been achieved!
en.wikipedia.org/wiki/British_Overseas_Territories#Current_overseas_territories
Nevertheless, post-independence flags can still be designed in advance as a way to promote the idea of independence.
Beyond this, the flags of all the former Western colonial powers need to come down someday.
I also want to draw attention to the flag of Myanmar:
en.wikipedia.org/wiki/Flag_of_Myanmar
We oppose the current flag (in use since 2010, following democratization):

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Democracy sucks:
www.nzherald.co.nz/nz/news/article.cfm?c_id=1&objectid=12242524National flag: Former PM John Key would now bypass referendum and just choose new ensign
...
Key - the current chairman of ANZ Bank NZ - had been strongly in favour of changing the national flag in 2015 and 2016 when he was Prime Minister, preferring a black and white silver fern design to the current flag with its British Union Jack on it.
His Government spent $26 million and two years engaging with the public in a bid to bring about the change.
Two referendums were held; one choosing the best flag design and a second to decide whether it should replace the current flag.
Eventually, 56.6 per cent of Kiwi voters chose to keep the old flag in March 2016, compared with 43.2 per cent in favour of a new flag design.
Key was criticised for the cost of the process and starting a contentious debate that divided the country.
But he told 66 Magazine he would now bypass and do away with the debate and go straight for the flag change because New Zealand needed an iconic symbol.
"I still believe that, as a small country at the bottom of the world if we want people to know us, we need a symbol that is ours," he said.
Yes. But the country also needs to stop being called "New Zealand", because it is undignified for a country in the Pacific Ocean to be named after a Dutch province:
https://trueleft.createaforum.com/issues/name-decolonization/Also, what sense does it make to change the flag while retaining the British head of state?
en.wikipedia.org/wiki/New_Zealand#Government_and_politics
Elizabeth II is the Queen of New Zealand[67] and thus the head of state.[68]
And most importantly, democracy should be scrapped. What sense does it make to change the flag while retaining a Western system of governance?
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Common sense:
www.news24.com/SouthAfrica/News/apartheid-flag-ruling-gratuitous-display-constitutes-hate-speech-20190821The gratuitous display of the old apartheid flag constitutes hate speech, the Equality Court, sitting in the Gauteng High court in Johannesburg, ruled on Wednesday.
The Nelson Mandela Foundation and Human Rights Commission took the case to court, asking that the gratuitous display of the flag be stopped.
Handing down judgment on Wednesday, Judge Phineas Mojapelo said the display of the old flag gratuitously demeans and dehumanises on the basis of race. He added that it impairs dignity.
I am against the inaccurate term "hate speech" (why not simply "colonialist speech"?), but the ruling itself is a step in the correct direction. The whole world should folllow South Africa's example and take it further. Displaying any colonial flag (including the flag of Israel) without countervailing context (e.g. burning it) should be regarded as a declaration of war by default, and displayers treated as enemy combat units.