Should one (almost) always use English for software implementation and documentation?

I would generally recommend using English whenever possible.

That said, there is one practical exception: if your project contains domain-specific terms, names, or concepts that don’t translate well (or would sound unnatural when translated), it’s perfectly reasonable to keep those in the native language. Forcing translations in such cases often reduces clarity instead of improving it.

However, mixing languages arbitrarily is where things become problematic.

From experience, one of the most frustrating situations is inheriting a project that uses multiple languages inconsistently. You end up mentally translating identifiers, comments, and documentation just to understand what’s going on. This adds unnecessary cognitive load and slows everything down.

It becomes even worse when new developers join who don’t speak the original language — onboarding becomes harder, and mistakes are more likely.

If you struggle with naming, it’s better to use simple English (even imperfect) or a translator than to fall back to another language.

In practice:

• Use English as the default language.

• Only use native terms if the translation would harm clarity.

• Avoid mixing languages within the same codebase.

Agree with what @Klaus said, so here's just one add-on:

If you use domain specific terms, which can be translated, please do so and maintain a glossary of such terms such that they can be looked up in their original language. Ideally such a glossary could be web-accessible, and code could contain hyperlinks in comments.

I know this is not a small task, requires discipline and ongoing maintenance, and the result might succumb to bit rot if the infrastructure hosting such a tool changes without being aware that this is a vital part of the documentation. But especially when considering larger codebases, larger teams, or external developers who might jump in at some time, it will likely pay back.

In a project that was originally designed with German names for the concepts, part of the work on analysis and reporting tasks is now performed by Indian colleagues whose English is very good, but of course the German words need to be translated to convey the meaning. Doing this on the spot when talking about a piece of code or a database table is adding mental load.

It depends. For me am writing my code and my documentation in code is 99% in English. The documentation in the readme is german since it is what my German speaking endusers come in contact with. Also all user facing strings when i code a frontend are in german but the names of the strings functions etc are in english. Usually coding in english is easier for me since the documention that read for different libreries is also english.