You know what I struggle with? Writing professional letters (or email) in my native language. I went to university and worked in the industry in the UK and so I only ever had to write in a professional style in English. I was trying to ask for some information from an online shop in Greek the other day (I'm Greek) and I found I had no idea how to start and end the email and how exactly to phrase the question I wanted to ask without sounding like I was talking to my bestie.

Kind of the reverse situation than what you're describing, because I haven't learned to say those things in my native language, but for me it goes to show we learn lots of pre-baked turns of phrase that we adjust appropriately to the context when we need them.

That's interesting because you're right, in English there are different voices depending on the context and who you're talking to. Is it a professional context or casual or if you're supposed to be doing something important or tactful.

In some countries, this is extended to even using different languages depending on the context, especially more in the Eastern countries.

This is only a problem when translating words or short phrases which is usually one of the first things that language learners try to do.

When you know the original intent behind a sentence (for example if you are its author), you can usually convey its meaning it in any of the languages you know well.

As your wife said, there is a way to express a friendly sentiment to somebody on their anniversary and let them know that you remember them. It's just not literally using the words "Happy Birthday".