How can I either insert Spanish letters or switch alphabets?
First solution: menu Insert → Special characters, then choose from various subsets of Unicode.
This is not however user-friendly if you have many such characters to enter, e.g. you are writing a Spanish text. In this case, if you have the possibility, a more practical solution uses the versatile input features of modern OSes:
Second solution: plug a Spanish keyboard to your computer and tell the OS about character lay-out.
(A surrogate alternative to this one is to use your usual keyboard after annotating the keys with the generated Spanish characters)
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