1. Download Glossary Converter
You need to have a SDL Account with a Studio license to download it for free.
In Glossary Converter you have to click on Settings and select for "Output format when converting any input" TBX (Term Base eXchange)
And tick "Use the selected output format for any input format"
2. Go to Settings and select the options as in the print screen below to get a TBX from an xlxs
3. Drag and drop the Excel file in the Glossary Converter window
The Excel file needs to have 2 columns and the first row contain the source and target languages.
4. Use the resulting .tbx file in your Language Cloud Dictionary.