Hi again Jonas, and thanks to German for providing a more detailed explanation than mine.
The case for having all translations on AMOS rather than in a lang folder inside your plugin files is as follows.
After you have put your plugin in the Moodle plugins repository, if you realize there is an error somewhere in your translated strings OR you have forgotten to translate a string...
a) if your Swedish lang strings are inside a yourmodule/lang/se/ folder, then you would have to edit/add those strings and commit a new version of your module to the repository, and tell your Swedish users to upgrade their module to see the new strings.
b) On the contrary, if there is no yourmodule/lang/se/ folder in your module in the Moodle plugins reposiroty but all Swedish strings are on AMOS, once you have edited the strings (and asked the Swedish lang maintainer to accept your translation), then all the end-user has to do is to update their Swedish language pack from within their Moodle site admin interface.
Note.- If you have a set of translated strings both inside your plugin lang folder and on AMOS, the AMOS translations take precedence over the strings in the local folder. Which means that, in my case (a) above, you might still edit/add strings on AMOS and your end-users would only have to update their Swedish lang pack. But really it does not make sense to have translated strings in 2 places (in a local lang folder and on AMOS). Having all translations on AMOS is really the best solution.
I hope this makes things clear,
Joseph