require a language for invoice templates

[kevinpapst]

Description

Preparation for 2.0

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I verified that my code applies to the guidelines (composer code-check)
• I updated the documentation (see here)
• I agree that this code is used in Kimai and will be published under the MIT license

[codecov]

Codecov Report

Merging #3387 (dd31438) into master (f30bb89) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #3387      +/-   ##
============================================
- Coverage     91.89%   91.85%   -0.04%     
  Complexity     7810     7810              
============================================
  Files           742      742              
  Lines         23599    23599              
============================================
- Hits          21687    21678       -9     
- Misses         1912     1921       +9     

Impacted Files	Coverage Δ
src/Form/InvoiceTemplateForm.php	100.00% <ø> (ø)
src/Controller/QuickEntryController.php	67.24% <0.00%> (-7.76%)	⬇️