Language Influences Thought

>>there is a theory called "semantic completeness" that states that any thought can be expressed in any language, <<

In theory, yes. In practice, each language encodes and facilities expressing those concepts that its speakers value and use the most. Conversely, it will have fewer or no words and features for things they don't talk about much or at all. So for some words, there is a direct translation, while others require several words. And then there are concepts that go against the grain of the language, or are themselves based on concepts that also don't exist in the language, or rely on semantic features the language doesn't have. That's where you get into translations that take a page of explanation or 5+ minutes of discussion. IF your audience can follow all that, and doesn't just get bored and change the subject.

>> with the implication that any language can expand to include any new notions. <<

This is possible, but it doesn't always happen. That particular issue is cultural rather than linguistic: a language is a living growing thing, but a culture can get stubborn and refuse to change. Some cultures are more inclined to embrace change, including linguistic change. Others are downright stuffy.

>> i daresay japanese speakers would be a mite insulted at your implication that their language lacks subtlety <<

It's not a matter of lacking subtlety, but rather, a difference in design. Japanese is a very structured, very precise language. There is often one right way to say something, based on rules that dictate which out of a bunch of similar-seeming words is appropriate to the occasion. That's often based on politeness, which is a prevailing feature in Japanese, although there are other groupings -- some words that only men use, or only women use, or are only used to describe either men or women.

Translating from Japanese to English can be challenging because Japanese encodes a lot of social dynamics into the grammar and word choice, whereas English pretty much does that only with word choice and once two people are using the same register even that doesn't signal much. You can still express similar ideas but doing so in English is often clunkier and lengthier. It's the difference between having a proper cherry pitter and pitting cherries with a paring knife.