Hello,
Sure, you can add whatever language you want. The language list is just there to conveniently prefill input fields for some languages (generally those in which WordPress or Polylang are translated).
* The name is used for display.
* The locale is used to find translations files. The accepted rule is to use the language code and then if the language is spoken (in a different way) in several countries, add an underscore and the country code (in uppercase). So there is ‘fr_FR’, ‘fr_CA’ for French but ‘ja’ for Japanese.
* The language code will be used in urls. Although it is written like this, you don’t need to use ISO 639-1 (not all languages are in this list!). It is generally accepted to use ISO 639-2 code, when there is no ISO 639-1.
* Choose the text direction.
Both locale and language code can be used to detect the browser language (I don’t know if Syriac is in browsers languages list)
The locale is also used to name the flag if you use one. Don’t put a custom flag in wp-content/plugins/polylang/flags as it will be deleted at next update. Create a directory wp-content/polylang and put the flag in it.