The answer is actually geographical. Spanish, just like French, and Portuguese, among others, are romantic languages. This means, by consequence, that they have their roots on Roman Latin. Now, going back to the roman empire; Romans used a lot of words to describe the German tribes across time, and even some to describe the whole Germanic territory. Notably, during about the time of some of the roman expansions to what is now Spain and France (around the birth of the roman empire), they used the word "Alamania" to refer to broadly the germanic territory, solely because the Alemanni was the tribe that occupied territory closer to the Empire, and had the most contact with it, and that's the reason why "Alemania" and its variants are used to refer to Germany in Romantic Languages.
On the other hand, the word Germanen, from which "Germany" stems, is a much older word, meaning "On this side of the Rhine" (the river Rhine), which at that time was used to refer ro a completely separate group of people, the (in latin) "Eburones", who inhabited what is now Belgium.

(BTW, sorry if my english is a bit broken. My native language is Spanish. Hope the answer helps)
Edit - thanks /u/TheDreadfulSagittary for correcting my spelling of the word "Rhine"

In the event that you don't find the answers you're looking for here, have you thought about cross-posting this question to /r/etymology?

Different etymological source. A lot of languages from areas of former roman influence base their word for it on the latin Germania. I would wager the "Alemania" comes from the french word for Germany which is "Allemagne". Since the French claim to be succesors to Franks, themselve a germanic tribe that took over rule over the romanized Gauls, they can't really call them germans since thas what they technically are themselves. The word for the only border region not inhabited by other frankish tribes was where the Alemanni lived(Today parts of Alsace, Baden-Würtemberg, Switzerland and Bavaria) and was expanded to cover all of Germany later on.

"Deutschland" does not mean land of the German people, as the word "Deutsch" only later changed to be used as a self descriptor devoid of the original meaning, but instead means "Land of those that belong to us" as the various variations of the modern "deutsch" in the different Dialects meant "belonging to your own people". (Just to be clear, this is an awkward translation because English lacks a proper word to translate stuff like "Volk" to, which a LOT of other languages have, which is also why "Peoples Republic" is such an akward translation because it actually means something different. This word instead developed into the word "folk" in English, which means something distantly similar, but really completely diffrent)

"Ludwig the German"("Ludwig der Deutsche") was not really "the German" but "one of us"/"belonging to the people" for example.

Edit: Fun fact: The word for german in english used to be alemain as well. That changed when the Dutch and German started to act more independently as alemain described both of them together. German only really replaced Alemain in the 16th century.

Fun fact 2: In Finnish and Estonian they are called the Saxons for the same reasons as they are called the Alemani in Spanish.

Fun fact 3: In slavic languages the Germans are called "The Foreigners", literaly "The Mute" because those barbarians could not speak a proper language slavs could understand.

Fun fact 4: Barbarian pretty much means the same, as "Barbarian" was meant to describe all those uncivilized people that could not speak a civilized language such as glorious Greek, and thus could only say uncomprehensible "Bar bar bar bar bar" all day.

Fun fact 5: In almost all languages (including Chinese, Japanese, Vietnames...) that did not have a need for a word for Germany until after Germany allready had it's own name, it is named as close to "Deutschland"/"Deutsch" as possible in those languages.