This is how to select your languages in Firefox (sorry for the pictures : I have only the french version of Firefox ; if somebody sends me the pictures in English I shall be pleased to integrate them).

First of all, please call the Tools menu, and the Options command.

The Options dialog box appears :

After clicking on the Choose button, you get a language list dialog box :

It appears to me that the rest is intuitive. Do not hesitate to tell me if it is not.

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