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Finally, I've downloaded the extension from BabelZilla, installed it in a new Firefox profile, and the Hungarian locale worked correctly (I haven't test it in French).
Yes, the hungarian description works OK.
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Please, try to remove NewTabURL, restart Firefox and install it again. If it still doesn't work, try to create a new profile and install it there, so see if it's a problem with NewTabURL or with Firefox... :-(
Yes, the hungarian description works OK.
But: if You download the hungarian locale from BabelZilla and if you replace the original main.properties file in hu-HU locale with downloaded locale file from BZ, the description is in English.
I see, that You upload the new version into Your homepage.
The Hungarian description works OK (the hu-HU locale files are what I send you as attachment).
I'm sure, that some other locale descriptions works not.