Caveat to customise Ubuntu locale for Ubuntu Thunderbird

Further to my post on how to customise the locale in Ubuntu and my suggestion to use the following format:
     xx_YY.CHARSET@extra

as a way to provide customised locale settings without replacing the system's.

Unfortunately, Ubuntu Thunderbird (ver 17.0.6) cannot handle the above mentioned convention even though Ubuntu can comfortably handling this.

By way of experimentation, I have discovered that it appears that Thunderbird picks up the local specifier in the LC_* environment variables and drops the part from @ onward and then using that as the locale specifier to load the locale settings.

So for example if you customise say English (Hong Kong) and name the customisation where you change the time/date format as en_HK@test and then you select this in your Regional Formats. When Thunderbird loads up, it will pick up the specifier en_HK.utf-8@test but it will only use en_HK.utf8 to load up the appropriate locale settings thus ignoring your customisation.

Hence if you want to correct the locale for Thunderbird, the only way is to replace the system file.