When downloading translations the plugin doesn't take any custom locales into account

[frett]

OneSky allows you to set a custom locale for a language:

When downloading translation files from the OneSky website it respects this custom locale, but the plugin currently uses the original locale.

It appears that the fetch project languages API has an undocumented custom_code attribute that is returned for languages.

Sample response from one of our projects:

        {
            "code": "lv-LV",
            "custom_locale": "lv",
            "english_name": "Latvian (Latvia)",
            "is_base_language": false,
            "is_ready_to_publish": false,
            "last_updated_at": "2024-03-28T20:38:38+0000",
            "last_updated_at_timestamp": 1711658318,
            "local_name": "Latviešu (Latvija)",
            "locale": "lv",
            "region": "LV",
            "translation_progress": "100.0%"
        },
        {
            "code": "te-IN",
            "custom_locale": null,
            "english_name": "Telugu (India)",
            "is_base_language": false,
            "is_ready_to_publish": false,
            "last_updated_at": "2023-12-29T22:22:27+0000",
            "last_updated_at_timestamp": 1703888547,
            "local_name": "తెలుగు (భారత దేశం)",
            "locale": "te",
            "region": "IN",
            "translation_progress": "59.4%"
        },

It would be nice if the plugin respected the custom_locale when downloading a translation.