Unfortunately, this subject is way too complicated. ;) I know, I've done the research as well.
To get you started though,
create a Resources directory in your assembly.
Start with English and add a "Resources File" (.resx) to that directory. Name it something like "text.resx". In the event that the localized resource can't be found, the app will default to pulling out of this file.
Add your text resources.
Add another resources file. Name this one something like "text.es.resx" Note the "es" part of the file name. In this case, that defines spanish. Note that each language has it's own character code definition. Look that up.
Add your spanish resources to it.
Now that we have resource files to work from, let's try to implement.
In order to set the culture, pull that from your database record. Then do the following:
String culture = "es-MX"; // defines spanish culture
Thread.CurrentThread.CurrentCulture = CultureInfo.CreateSpecificCulture(culture);
Thread.CurrentThread.CurrentUICulture = new CultureInfo(culture);
This could happen in the app that has loaded your assembly OR in the assembly initialization itself. You pick.
To utlize the resource, all you have to do is something like the following within your assembly:
public string TestMessage() {
return Resources.Text.SomeTextValue;
}
Ta Da. Resources made easy. Things can get a little more complicated if you need to change usercontrols or do something directly in an aspx page. Update your question if you need more info.
Note that you could have resource files named like "text.es-mx.resx" That would be specific to mexican spanish. However, that's not always necessary because "es-mx" will fall back to "es" before it falls back to the default. Only you will know how specific your resources need to be.