Slán has many meanings, one of which is "bye". sigmacharding and NiallT are correct that it also means "safe" (save) and "health", respectively. It would be helpful if the feedback to a question would elaborate on multiple translations of a word rather than indicating an incorrect answer when a responder has chosen an alternative translation.
If you're using the web, the accented characters are displayed below the text box - click on them to add the accents, or just enable accented characters on your keyboard - it's a software issue, not a hardware issue.
If you're using an app on a phone, just long-press on a vowel to get the option of an accent.
Accents are not optional in Irish. If you're not using them, you're not spelling the words correctly.
Nope! If you are seeing a word for the first time it should be highlighted with yellow amd have a broken underline. You just need to tap (in the app) or hover over (using desktop) the word and it will show a translation for the word.
Unless you are in 'test out' sessions, or it is offering you pictures to offer context.
Hope that helps :)