Project owners and administrators can change a language in a POEditor project to another in the top right menu of the respective language page, by clicking on Change Language.

Then choose the language you want to switch to from your language menu and click on the Save button. Note that the translations for terms with plural forms will be lost (including the singular form) if you switch to another language.

If you want to switch to a custom language, you need to add the custom language to the language menu first. To do this, simply add the language to the project using the Add language button, as you would add any new language. Then, even if you delete the custom language, POEditor will memorize it as an option in your language menu.