Were Allies of WWI called "Allies" at the time (in English), or is that a term we started applying later? Did they just refer to themselves as the (Triple) Entente?
Yes, at the start of the war. No, by the end.
It's an interesting question because the "Allies" were the "Entente Cordiale" of the UK and France and later the "Triple Entente" when Russia joined. They were formed to counter the "Triple Alliance" of Germany, Austria-Hungary, and Italy.
At the start of the war, yes, using the term "Allies" would be an anachronism. They were "the Entente". But by the end of the war, the coalition fighting the Central Powers were referred to as "the Allied Powers" in part because the Triple Alliance collapsed (Italy remained neutral and then declared war on Austria-Hungary) and there were just so many nations fighting the Central Powers most of whom were not members of the Entente.
The term "alliance" was very deeply embedded into the 19th century. Nowadays it's been replaced by the word "collation". There was the Dual Alliance, the Franco-Russian Alliance, there was even an Austro-Serbian Alliance.
At the time of The Great War allies fought allies, you're right, but in hindsight, mostly from a popular standpoint, it was easier to distinguish the two sides. Since the Triple Alliance broke up and the Triple Entente drew in more and more and more countries to their cause it was easier to simplify the term.
One example is that the United States was not technically an "ally" but rather an "associated power" who declared war on Germany for violating their neutrality. They wished to avoid "foreign entanglements" and so did not want to get involved in a European alliance.
The countries fighting the Central Powers are referred to in treaties as "The Allied Powers" or "The Allied and Associated Powers". For example there are...
- "Treaty of Peace between the Allied and Associated Powers and Germany" (aka the Treaty of Versailles)
- "Treaty of Peace between the Allied Powers and Austria"
- "The Treaty of Peace Between the Allied Powers and the Ottoman Empire" of 1920.
- "Treaty between the Principal Allied and Associated Powers and Bulgaria and Protocol"
- "Treaty of Peace between the Allied and Associated Powers and Hungary"