Hi, 
I use POI 3.17 version. 
And i found that the formated date cell for instance allways follow the current locale. 

Analysis : 

With "this.localeIsAdapting = true;" in the constructor with 3 arguments 
we can never set localsAdapting to <code>false</code> in the Construstor with 2 arguments. 

it will always be <code>true</code> for field localIsAdapting. 
 
in  "this(locale, false, emulateCSV);"


//snipppet org.apache.poi.ss.usermodel.DataFormatter -------------

    /**
     * Creates a formatter using the given locale.
     *
     * @param  emulateCSV whether to emulate CSV output.
     */
    public DataFormatter(Locale locale, boolean emulateCSV) {
        this(locale, false, emulateCSV);
    }

    /**
     * Creates a formatter using the given locale.
     * @param  localeIsAdapting (true only if locale is not user-specified)
     * @param  emulateCSV whether to emulate CSV output.
     */
    private DataFormatter(Locale locale, boolean localeIsAdapting, boolean emulateCSV) {
        this.localeIsAdapting = true;
        localeChangedObservable.addObserver(this);
        // localeIsAdapting must be true prior to this first checkForLocaleChange call.
        localeChangedObservable.checkForLocaleChange(locale);
        // set localeIsAdapting so subsequent checks perform correctly
        // (whether a specific locale was provided to this DataFormatter or DataFormatter should
        // adapt to the current user locale as the locale changes)
        this.localeIsAdapting = localeIsAdapting;
        this.emulateCSV = emulateCSV;
    }


Best Regards,
Rochemont Patrice.