I am sorry for your loss.

You must declare the full inherited amount as assets in the calendar year in which the deceased passed away even if the funds were not yet distributed. If the inheritance includes fixed assets such as property, you need a valuation for that as well.

Chances are the documentation you will receive is in English, which is not the problem, except you will need to enter it into the tax form where the wording might be a bit tricky. If you are not the sole inheritor, you need to show the full amount and declare your share.
Backup documentation is important.

According the Basel Kantonalbank, in Basel Land you pay tax on the inheritance from a sibling (but not from parents). For Baselstadt the situation is the same. Thought this is worth noting as the OP said "first degree relative" but was not specific.

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