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I've received many request to add a locale switching functionality. Moreover, many users failed to set up Gmail IMAP due to a mismatch between Thunderbird and Gmail language settings. So, I've implemented the locale switching in Gmail IMAP Account Setup 1.6.
"Language of Gmail" setting is added in the first page of the account setup wizard. This option is enabled when you choose "Gmail IMAP" or "Gmail IMAP (Google Apps). The locale which is the same as the one Thunderbird uses is showed by default. If it differs from Gmail's language setting, choose the correct locale. For example, select "en-US" if your Gmail setting is US English. If your locale is not showed in the list, it is not supported so far. The locale switching requres restart of Thunderbird. Continue the account setup after Thunderbird was restart.
Newly added strings have not been localized yet because v1.6 is an experimental release.
Please let me know if you find any problems.