How do I shorten a translation that does not fit the UI? 

Start with the limit, not the text. Give the string key a maximum length in the editor and the length check flags every language that exceeds it, so you fix all of them instead of the one someone noticed. German, French and Finnish run 20 to 35 percent longer than English, so a button sized for the English text overflows in several languages at once.

Then shorten the translations that fail, in this order:

1. Cut filler. "Please enter your email address" becomes "Enter your email" in most languages without losing anything.
2. Use the target language's own short forms. Many languages have standard UI abbreviations and shorter synonyms that a translator knows and a dictionary does not list. Ask the translator for the version they would put on a button, not the one they would put in a manual.
3. Ask the AI Assistant. In Localizely, the AI Assistant has a Shorten to N preset that rewrites the translation within the key's limit, keeps every placeholder, and shows the difference before you insert it. If the limit cannot be met, it says so instead of guessing.
4. Change the design, not the words. When even the shortest honest translation does not fit, the button is too small. Wrapping, a smaller font for long languages, or an icon with a tooltip are better than a mistranslation.

Avoid abbreviating by truncation ("Registr…"), dropping articles in languages that need them, and squeezing text with narrower letter spacing. All three read as broken to native speakers.