By default this is probably set to American. Change it as follows:
Single left-click the Thunderbird menu button (the three horizontal lines at top right) - a small box will open.
Hover the mouse pointer over "Options" (in Windows) or "Preferences" (in Linux) - another small box will open.
Single left-click "Options" (in Windows) or "Preferences" (in Linux) at the top of this box - a bigger "Thunderbird Preferences" box will open.
Single left-click the "Composition" button and then the "Spelling" tab.
In the middle you will see "Language: English/United States".
Single left-click the tiny triangle at the right hand end of that line and then single left-click the language that you want.
Single left-click the "Close" button in the bottom right-hand corner of the "Thunderbird Preferences" box.