There's no such thing as "untranslatable" between languages. Sometimes words are elevated to these lofty positions for no good reason, we Welsh have hiraeth which is also said to be untranslatable, but it pretty much means "longing/yearning".
Bad and poor translations are only due to the translators poor ability.
EDIT: Those who downvoted clearly know nothing of languages and how they work.
True. Usually it just means that there isn't a directly equivalent word in the target language. Often a set of different words can be used depending on context or sometimes multiple words are needed.
Like Swedish "lagom" translates to "just enough for the purpose". It is very much translateable, but doesn't have one specific English word it translates to.
Same with "sisu". "Stoic grit" would be a good translation in many contexts.
@Jonlang_: Don't forget that speaking in any language the speaker already has to "translate" his internal thoughts into spoken words and body language, and in that transliteration sometimes "untranslatability" plays a role. If you would study sign language, i.e. the (tactile) hand signs in order to "speak" with a deaf/deaf+blind person, this aspect of communication becomes much more important. BTW, sign languages are real languages of its own, with grammar and all linguistic aspects like spoken (and sung) languages have. However, the grammar of a sign language usually does not compare at all to that of the spoken language in that country. (Aug 2020)
Sisu is a Finnish concept described as stoic determination, tenacity of purpose, grit, bravery, resilience, and hardiness and is held by Finns themselves to express their national character. It is generally considered not to have a literal equivalent in English.
That description is mor or less accurate. There does not appear to be an exact translation into English. Courage isn't an exact translation. Fortitude might be a better translation. However, sisu seems to imply courage, fortitude and resilience. (There is, in general, nowhere near a one-to-one correlation between languages.)
Love that they've used a bit of purely Finnish phrasing here. Very much doubt I would ever use this phrasing, but certainly good to learn!
English is a rich language and inventing a new word such as 'sisu' seems questionable. Especially since the concept is difficult to follow for people who have no previous encounter with Finnish. The word comes up at a very early level dedicated for beginners. Yet there are so many great words in Finnish that can be properly translated, why not to focus on those instead? That's just my subjective opinion, and I wish you good luck!
I would translate it as "pluck". It's not entirely equivalent but it's close enough, and the same can be said of many commonly used translations. In my view, the only good reason to do an avoidable loan translation is when the cultural transfer from the loan translation is deemed more valuable for the purpose of the translation than maximising the intelligibility of the translation. This is generally the case when there are stylistic purposes involved, so I'm personally not sure whether insisting on a loan translation as the only option is beneficial for purely pedagogical purposes.