When I type a Polish letter such as ł, ą, ę, etc. in an invoice field and save the invoice, the letter is deleted.
However, in the translated Polish headers on the invoice line item table, Polish letters do appear properly.
Unfortunately there are certain fields that only support non-diacritic characters, the invoice line description field is one such example. Without getting too technical this was an early design decision taken back in 2010 that is not simple to change.
I will discuss this with our database administrator but I’m afraid it is not likely to be something we can address in the short term.
I understand the limitation — thank you for clarifying.
Meanwhile, is it possible to have Polish diacritic characters render automatically as their non-diacritic counterparts, i.e., ‘ą’ as ‘a’, etc.?
If that’s too much work, it’s no big deal — I’ll just have to adjust my spelling 
and wait for you to upgrade your core technology.