plugin wordpress .po switch

Hello I needed to make a plugin that would switch language from pre-created .PO files. primarily a language file template and several translated plugins. So if I set up in administration eg en_GB, I have everything in English. If cs_CZ is all Czech. it works ok. But I needed this to be set up individually by the user directly on the web. and the language would be set by default by browser. Furthermore, one field for "description" posts could be translated individually for all languages. eg so that in the administration, when editing the post, the field would appear in the tab and the different tabs would be in the corresponding language. If the user writes a post, this "description" field should be put into the tab of the language the user uses. Switch should do so that it can be inserted into the wp menu language name + flag drop-down list.