EU-wide standards and technical specifications exist for a number of product categories. Where EU specifications exist for your product(s), your products must comply with them before you can trade them freely in the EU.
Where no EU-wide specifications exist, different specifications might apply in different EU countries. In such cases, you must comply only with the rules valid in your country. Other countries cannot forbid the sale of your product nor oblige you to modify it or do additional testing, provided you can prove that your products fulfil all the technical and quality requirements in your own country and offer a similar level of safety: this is the principle of mutual recognition.
In particular, national rules might differ on requirements relating to:
If you sell to other businesses - and not to final consumers - there is no obligation to use a given language for your products. For sales to final consumers, you might be obliged to use the national language(s) of the country you want to export to.
If your products pose a risk to the public interest, you might not be allowed to export them freely. Your products could be forbidden if the country you want to export to can prove that its own technical requirements are essential or mandatory, and your products are not equivalent in terms of the level of safety sought.
Dig deeper, country by country: